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Sample records for early proterozoic rocks

  1. Results of paleomagnetic study of Early Proterozoic rocks in the Baikal Range of the Siberian craton

    NASA Astrophysics Data System (ADS)

    Vodovozov, V. Yu.; Didenko, A. N.; Gladkochub, D. P.; Mazukabzov, A. M.; Donskaya, T. V.

    2007-10-01

    This paper presents paleomagnetic results obtained from the study of Early Proterozoic rocks in the Baikal Range of the Siberian craton, namely, the 1850 1880-Ma volcanicalstic rocks of the Akitkanskian series of the North Baikal volcanic-plutonic belt) and 1674-Ma basic dikes of the Chaya complex within the massif. The data of this work are used to reconstruct the development of the Siberian craton structure in the Early Precambrian. The projections of the inferred paleomagnetic directions onto a sphere form S (southern) and W (western) groups of vectors of characteristic magnetization components. The S group consists of three clusters representing primary magnetization components belonging to different time levels of the end of the Early Proterozoic. The W group is represented by directions associated with a metachronous magnetization probably acquired during the Riphean. Four paleomagnetic poles are obtained. Two of them that can be regarded as key poles correspond to time levels of 1875 and 1670 Ma (the Early Proterozoic). The two other poles can be used for a detailed reconstruction of the Proterozoic segment of the Siberian apparent polar wander path. The data presented in the paper indicate that the formation of the southern Siberian craton structure was accomplished at the end of the Early Proterozoic, which resulted in a synchronous motion of different blocks composing the southern flank of the craton (in particular, the Sharyzhalgai and Baikal Ranges).

  2. Early Proterozoic crustal evolution: Geochemical and NdPb isotopic evidence from metasedimentary rocks, southwestern North America

    NASA Astrophysics Data System (ADS)

    McLennan, S. M.; Hemming, S. R.; Taylor, S. R.; Eriksson, K. A.

    1995-03-01

    Early Proterozoic (1.8-1.7 Ga) metasedimentary rocks in northern New Mexico and southern Colorado, USA, can be divided into turbidite successions (commonly volcanogenic) associated with mafic/felsic metavolcanic successions (e.g., Irving Fm.) and stable shelf quartzite-pelite successions of shallow marine origin (e.g., Hondo Gp.). Metapelites from the turbidite successions reported here have low K2O/Na2O, low Th/U (<3.0), low to moderate Th/Sc (0.1-0.6), and slight negative Eu-anomalies, although regionally, negative Eu-anomalies in such rocks are common. At the time of sedimentation (ca. 1.7-1.8 Ga), ɛNd values were in the range +3 to +7, indistinguishable from associated metavolcanic and plutonic rocks. Similarly, lead isotopic data scatter about a 1.7 Ga reference isochron. Low κ (232Th/238U) values for the Irving Formation are consistent with derivation from crustal sources similar to the southern Colorado/northern New Mexico lead isotope crustal province. These data are further consistent with a volcanic arc related origin. In contrast, stable shelf metapelites have high K2O/Na2O, variable but commonly high Th/U (2.0-7.0), moderate to high Th/Sc (0.5-1.4), and substantial negative Eu-anomalies. Although compositions are rather variable, they are typical of post-Archean shales. Neodymium isotopes are surprisingly radiogenic with ɛNd(1.7 Ga) in the range -0.2 to +4. Lead isotopic data for the least radiogenic samples also are consistent with a dominantly juvenile source and on a 207Pb/204Pb vs. 206Pb/204Pb diagram, data scatter slightly above the 1.7 Ga reference isochron, suggesting minor components of significantly older material. Lead isotopic systematics suggest that a major component of the provenance was derived from the immediately associated metavolcanic-plutonic terranes, consistent with suggestions of a first-cycle origin, but with an Archean component. Isotopic data restrict the Archean component to about 10%, on average, and no more than 25% in

  3. A reconstruction of Proterozoic rocks in north-central New Mexico: Tectonic implications from the Proterozoic to the Cenozoic

    SciT

    Daniel, C.G.; Karlstrom, K.E.

    1993-04-01

    Distinctive lithostratigraphic markers, metamorphic isobaric surfaces, major ductile thrusts and overturned folds in Early Proterozoic rocks from 4 isolated uplifts in north-central NM provide relatively firm piercing points for restoration of over 50 km of right lateral strike-slip movement along a network of N-S trending faults. In addition, the authors speculate that the Uncompahgre Group in the Needle Mts. of southern Colorado is correlative with the Hondo Group in northern NM; suggesting over 150 km of right-lateral strike slip offset has occurred across a network of N-S trending faults that includes the Picuris-Pecos fault, the Borrego fault, the Nacimiento faultmore » and others. The tectonic implications of this reconstruction span geologic time from the Proterozoic to the Cenozoic. The restoration of slip provides new insights into the structure of the Proterozoic basement in NM. Volcanogenic basement (1.74--1.72 Ga) and overlying sedimentary cover (Hondo Group) are imbricated in an originally EW- to NW-trending ductile foreland thrust and fold belt that formed near the southern margin of 1.74--1.72 basement. The authors propose that the volcanogenic basement rocks correlate with rocks of the Yavapi Province in Arizona and that the Hondo Group correlates with foreland rocks of the Tonto Basin Supergroup. Rocks south of this belt are 1.65 Ga or younger and are interpreted to belong to a separate crustal province which correlates with the Mazatzal Province in Arizona. Proterozoic ductile fault geometries suggest that the Mazatzal Province was thrust northward and resulted in imbrication of Yavapi Province basement and its siliciclastic over sequence.« less

  4. Tectonics and metallogenesis of Proterozoic rocks of the Reading Prong

    Gundersen, L.C.S.

    2004-01-01

    Detailed geologic mapping, petrography, and major and trace-element analyses of Proterozoic rocks from the Greenwood Lake Quadrangle, New York are compared with chemical analyses and stratigraphic information compiled for the entire Reading Prong. A persistent regional stratigraphy is evident in the mapped area whose geochemistry indicates protoliths consistent with a back-arc marginal basin sequence. The proposed marginal basin may have been floored by an older sialic basement and overlain by a basin-fill sequence consisting of a basal tholeiitic basalt, basic to intermediate volcanic or volcaniclastic rocks and carbonate sediments, a bimodal calc-alkaline volcanic sequence, and finally volcaniclastic, marine, and continental sediments. The presence of high-chlorine biotite and scapolite may indicate circulation of brine fluids or the presence of evaporite layers in the sequence. Abundant, stratabound magnetite deposits with a geologic setting very unlike that of cratonic, Proterozoic banded-iron formations are found throughout the proposed basin sequence. Associated with many of the magnetite deposits is unusual uranium and rare-earth element mineralization. It is proposed here that these deposits formed in an exhalative, volcanogenic, depositional environment within an extensional back-arc marginal basin. Such a tectonic setting is consistent with interpretations of protoliths in other portions of the Reading Prong, the Central Metasedimentary Belt of the Canadian Grenville Province, and recent interpretation of the origin of the Franklin lead-zinc deposits, suggesting a more cohesive evolving arc/back-arc tectonic model for the entire Proterozoic margin of the north-eastern portion of the North American craton. Published by Elsevier Ltd.

  5. Hydrocarbon source-rock evaluation - Solor Church Formation (middle Proterozoic, Keweenawan Supergroup), southeastern Minnesota

    Hatch, J.R.; Morey, G.B.

    1984-01-01

    In the type section (Lonsdale 65-1 core, Rice County, Minnesota) the Solor Church Formation (Middle Proterozoic, Keweenawan Supergroup) consists primarily of reddish-brown mudstone and siltstone and pale reddish-brown sandstone. The sandstone and siltstone are texturally and mineralogically immature. Hydrocarbon source-rock evaluation of bluish-gray, greenish-gray and medium-dark-gray to grayish-black beds, which primarily occur in the lower 104 m (340 ft) of this core, shows: (1) the rocks have low organic carbon contents (<0.5 percent for 22 of 25 samples); (2) the organic matter is thermally very mature (Tmax = 494°C, sample 19) and is probably near the transition between the wet gas phase of catagenesis and metagenesis (dry gas zone); and (3) the rocks have minimal potential for producing additional hydrocarbons (genetic potential <0.30 mgHC/gm rock). Although no direct evidence exists from which to determine maximum depths of burial, the observed thermal maturity of the organic matter requires significantly greater depths of burial and(or) higher geothermal gradients. It is likely, at least on the St. Croix horst, that thermal alteration of the organic matter in the Solor Church took place relatively early, and that any hydrocarbons generated during this early thermal alteration were probably lost prior to deposition of the overlying Fond du Lac Formation (Middle Proterozoic, Keweenawan Supergroup).

  6. Hydrocarbon source rock evaluation: Solor Church Formation. (Middle Proterozoic, Keweenawan Supergroup) southeastern Minnesota

    SciT

    Hatch, J.R.; Morey, G.B.

    In the type section (Lonsdale 65-1 core, Rice County, Minnesota) the Solar Church Formation (Middle Proterozoic, Keweenawan Supergroup) consists primarily of reddish-brown mudstone and siltstone and pale reddish-brown sandstone. The sandstone and siltstone are texturally and mineralogically immature. Hydrocarbon source-rock evaluation of bluish-gray, greenish-gray and medium-dark-gray to grayish-black beds, which primarily occur in the lower 104 m (340 ft) of this core, shows: (1) the rocks have low organic carbon contents (<0.5% for 22 of 25 samples); (2) the organic matter is thermally very mature (T/sub max/ = 494/sup 0/C, sample 19) and is probably near the transition between themore » wet gas phase of catagenesis and metagenesis (dry gas zone); and (3) the rocks have minimal potential for producing additional hydrocarbons (genetic potential <0.30 mgHC/gm rock). Although no direct evidence exists from which to determine maximum depths of burial, the observed thermal maturity of the organic matter requires significantly greater depths of burial and(or) higher geothermal gradients. It is likely, at least on the St. Croix horst, that thermal alteration of the organic matter in the Solor Church took place relatively early, and that any hydrocarbons generated during this early thermal alteration were probably lost prior to deposition of the overlying Fond du Lac Formation (Middle Proterozoic, Keweenawan Supergroup). 5 figs., 2 tabs.« less

  7. The Wisconsin magmatic terrane: An Early Proterozoic greenstone-granite terrane formed by plate tectonic processes

    NASA Technical Reports Server (NTRS)

    Schulz, K. J.; Laberge, G. L.

    1986-01-01

    The Wisconsin magmatic terrane (WMT) is an east trending belt of dominantly volcanic-plutonic complexes of Early Proterozoic age (approx. 1850 m.y.) that lies to the south of the Archean rocks and Early Proterozoic epicratonic sequence (Marquette Range Supergroup) in Michigan. It is separated from the epicratonic Marquette Range Supergroup by the high-angle Niagara fault, is bounded on the south, in central Wisconsin, by Archean gneisses, is truncated on the west by rocks of the Midcontinent rift system, and is intruded on the east by the post-orogenic Wolf river batholith. The overall lithologic, geochemical, metallogenic, metamorphic, and deformational characteristics of the WMT are similar to those observed in recent volcanic arc terranes formed at sites of plate convergence. It is concluded that the WMT represents an evolved oceanic island-arc terrane accreated to the Superior craton in the Early Proterozoic. This conclusion is strengthened by the apparent absence of Archean basement from most of the WMT, and the recent recognition of the passive margin character of the epicratonic Marquette Range Supergroup.

  8. Geochemistry and stratigraphic relations of middle Proterozoic rocks of the New Jersey Highlands

    Volkert, Richard A.; Drake, Avery Ala

    1999-01-01

    Middle Proterozoic rocks of the New Jersey Highlands consist of a basement of dacitic, tonalitic, trondhjemitic, and charnockitic rocks that constitute the Losee metamorphic suite. These rocks are unconformably overlain by a layered supracrustal sequence of quartzo-feldspathic and calcareous rocks. Abundant sheets of hornblende- and biotite-bearing rocks of the Byram intrusive suite and clinopyroxene-bearing rocks of the Lake Hopatcong intrusive suite were synkinematically emplaced at about 1,090 Ma. These intrusive suites constitute the Vernon Supersuite. The postorogenic Mount Eve Granite has been dated at 1,020?4 Ma and is confined to the extreme northern Highlands.

  9. HYDROCARBON SOURCE ROCK EVALUATION OF MIDDLE PROTEROZOIC SOLOR CHURCH FORMATION, NORTH AMERICAN MID-CONTINENT RIFT SYSTEM, RICE COUNTY, MINNESOTA.

    Hatch, J.R.; Morey, G.B.

    1985-01-01

    Hydrocarbon source rock evaluation of the Middle Proterozoic Solor Church Formation (Keweenawan Supergroup) as sampled in the Lonsdale 65-1 well, Rice County, shows that: the rocks are organic matter lean; the organic matter is thermally post-mature, probably near the transition between the wet gas phase of catagenesis and metagenesis; and the rocks have minimal potential for producing additional hydrocarbons. The observed thermal maturity of the organic matter requires significantly greater burial depths, a higher geothermal gradient, or both. It is likely, that thermal maturation of the organic matter in the Solor Church took place relatively early, and that any hydrocarbons generated during this early phase were probably lost prior to deposition of the overlying formation.

  10. A note on coarse-grained gravity-flow deposits within proterozoic lacustrine sedimentary rocks, Transvaal sequence, South Africa

    NASA Astrophysics Data System (ADS)

    Eriksson, P. G.

    A widely developed, thin, coarse-matrix conglomerate occurs within early Proterozoic lacustrine mudrocks in the Transvaal Sequence, South Africa. The poorly sorted tabular chert clasts, alternation of a planar clast fabric with disorientated zones, plus normal and inverse grading in the former rock type suggest deposition by density-modified grain-flow and high density turbidity currents. The lower fan-delta slope palæenvironment inferred for the conglomerate is consistent with the lacustrine interpretation for the enclosing mudrock facies. This intracratonic setting contrasts with the marine environment generally associated with density-modified grain-flow deposits.

  11. Pellet microfossils: Possible evidence for metazoan life in Early Proterozoic time

    PubMed Central

    Robbins, Eleanora Iberall; Porter, Karen Glaus; Haberyan, Kurt A.

    1985-01-01

    Microfossils resembling fecal pellets occur in acid-resistant residues and thin sections of Middle Cambrian to Early Proterozoic shale. The cylindrical microfossils average 50 × 110 μm and are the size and shape of fecal pellets produced by microscopic animals today. Pellets occur in dark gray and black rocks that were deposited in the facies that also preserves sulfide minerals and that represent environments analogous to those that preserve fecal pellets today. Rocks containing pellets and algal microfossils range in age from 0.53 to 1.9 gigayears (Gyr) and include Burgess Shale, Greyson and Newland Formations, Rove Formation, and Gunflint Iron-Formation. Similar rock types of Archean age, ranging from 2.68 to 3.8 Gyr, were barren of pellets. If the Proterozoic microfossils are fossilized fecal pellets, they provide evidence of metazoan life and a complex food chain at 1.9 Gyr ago. This occurrence predates macroscopic metazoan body fossils in the Ediacaran System at 0.67 Gyr, animal trace fossils from 0.9 to 1.3 Gyr, and fossils of unicellular eukaryotic plankton at 1.4 Gyr. Images PMID:16593599

  12. Preliminary digital geologic map of the Penokean (early Proterozoic) continental margin in northern Michigan and Wisconsin

    Cannon, W.F.; Ottke, Doug

    1999-01-01

    The data on this CD consist of geographic information system (GIS) coverages and tabular data on the geology of Early Proterozoic and Archean rocks in part of the Early Proterozoic Penokean orogeny. The map emphasizes metasedimentary and metavolcanic rocks that were deposited along the southern margin of the Superior craton and were later deformed during continental collision at about 1850 Ma. The area includes the famous iron ranges of the south shore region of the Lake Superior district. Base maps, both as digital raster graphics (DRG) and digital line graphs (DLG) are also provided for the convenience of users. The map has been compiled from many individual studies, mostly by USGS researchers, completed during the past 50 years, including many detailed (1:24,000 scale) geologic maps. Data was compiled at 1:100,000 scale and preserves most of the details of source materials. This product is a preliminary release of the geologic map data bases during ongoing studies of the geology and metallogeny of the Penokean continental margin. Files are provided in three formats: Federal Spatial Data Transfer format (SDTS), Arc export format (.e00) files, and Arc coverages. All files can be accessed directly from the CD-ROM using either ARC/INFO 7.1.2 or later or Arc View 3.0 or later software. ESRI's Arc Explorer, a free GIS data viewer available at the web site: http://www.esri.com/software/arcexplorer/index.html also provides display and querying capability for these files.

  13. Microbial Remains in Middle Proterozoic Rocks of Northern Australia

    NASA Technical Reports Server (NTRS)

    Astafieva, Marina; Rozanov, Alexei Yu.; Hoover, Richard B.; Vickers-Rich, P.; Wilde, A.

    2004-01-01

    Investigation of the samples of the McArthur River complex ore deposit, one of the most zinc-lead m i n d provinces in the world, brings us to conclusion about the possibility of the biogenic origin of sulfides in McArthur River ore deposit and to make suppositions about the formation of the studied rocks in the photic zone of sea.

  14. The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

    Tosdal, R.M.

    1996-01-01

    Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru?? from the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171??20 Ma and 1158??12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (???1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Bele??n Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher presentday 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link

  15. River Valley pluton, Ontario - A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

    NASA Technical Reports Server (NTRS)

    Ashwal, Lewis D.; Wooden, Joseph L.

    1989-01-01

    This paper presents Nd, Sr, and Pb isotopic data indicating a late-Archean/early-Proterozoic age for the River Valley anorthositic pluton of the southwestern Grenville Province of Sudbury, Ontario. Pb-Pb isotopic data on 10 whole-rock samples ranging in composition from anorthosite to gabbro yield an age of 2560 + or - 155 Ma. The River Valley pluton is thus the oldest anorthositic intrusive yet recognized within the Grenville Province. The Sm-Nd isotopic system records an age of 2377 + or - 68 Ma. High Pb-208/Pb-204 of deformed samples relative to igneous-textured rocks implies Th introduction and/or U loss during metamorphism in the River Valley area. Rb-Sr data from igneous-textured and deformed samples and from mineral separates give an age of 2185 + or - 105 Ma, indicating substantial disturbance of the Rb-Sr isotopic system.

  16. Keivy Paraschists (Archean-Early Proterozoic): Nanobacteria and Life

    NASA Astrophysics Data System (ADS)

    Astafieva, M. M.; Balaganskii, V. V.

    2018-05-01

    Nanobacteria, buried in situ, were discovered in the Early Precambrian paraschists (Keivy, Kola Peninsula). It is suggested that occurrence of nanobacteria indicates that a biological factor played a role in the formation of enclosing rocks.

  17. Facies analysis of Late Proterozoic through Lower Cambrian rocks of the Death Valley regional ground-water system and surrounding areas, Nevada and California

    SciT

    Sweetkind, D.S.; White, D.K.

    Late Proterozoic through Lower Cambrian rocks in the southern Great Basin form a westward-thickening wedge of predominantly clastic deposits that record deposition on the early western shelf edge of western North America (Stewart and Poole, 1974; Poole and others, 1992). Regional analyses of geologic controls on ground-water flow in the southern Great Basin typically combined lithostratigraphic units into more general hydrogeologic units that have considerable lateral extent and distinct hydrologic properties. The Late Proterozoic through Lower Cambrian rocks have been treated as a single hydrogeologic unit, named the lower clastic aquitard (Winograd and Thordarson, 1975) or the quartzite confining unitmore » (Laczniak and others, 1996), that serves as the hydrologic basement to the flow system. Although accurate in a general sense, this classification ignores well-established facies relations within these rocks that might increase bedrock permeability and locally influence ground-water flow . This report presents a facies analysis of Late Proterozoic through Lower Cambrian rocks (hereafter called the study interval) in the Death Valley regional ground-water flow system - that portion of the southern Great Basin that includes Death Valley, the Nevada Test Site, and the potential high-level nuclear waste underground repository at Yucca Mountain (fig. 1). The region discussed in this report, hereafter called the study area, covers approximately 100,000 km2 (lat 35 degrees-38 degrees 15'N., long 115 degrees-118 degrees W.). The purpose of this analysis is to provide a general documentation of facies transitions within the Late Proterozoic through Lower Cambrian rocks in order to provide an estimate of material properties (via rock type, grain size, and bedding characteristics) for specific hydrogeologic units to be included in a regional ground-water flow model.« less

  18. Early Proterozoic activity on Archean faults in the western Superior province - evidence from pseudotachylite

    Peterman, Z.E.; Day, W.

    1989-01-01

    Major transcurrent faults in the Superior province developed in the Late Archean at the close of the Kenoran orogeny. Reactivation of some of these faults late in the Early Proterozoic is indicated by Rb-Sr analyses of pseudotachylite from the Rainy Lake-Seine River and Quetico faults in the Rainy Lake region of Minnesota and Ontario. Fault veins of pseudotachylite and immediately adjacent country rock at two localities yielded subparallel isochrons that are pooled for an age of 1947??23 Ma. K-Ar and Rb-Sr biotite ages register earlier regional cooling of the terrane at about 2500 Ma with no evidence of younger thermal overprinting at temperatures exceeding 300??C. Accordingly, the 1947??23 Ma age is interpreted as dating the formation of the pseudotachylite. Reactivation of existing faults at this time was caused by stresses transmitted from margins of the Superior province where compressional tectonic events were occurring. -Authors

  19. Sedimentary petrography of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

    NASA Astrophysics Data System (ADS)

    Schreiber, U. M.; Eriksson, P. G.; van der Neut, M.; Snyman, C. P.

    1992-11-01

    Sandstone petrography, geochemistry and petrotectonic assemblages of the predominantly clastic sedimentary rocks of the Early Proterozoic Pretoria Group, Transvaal Sequence, point to relatively stable cratonic conditions at the beginning of sedimentation, interrupted by minor rifting events. Basement uplift and a second period of rifting occurred towards the end of Pretoria Group deposition, which was followed by the intrusion of mafic sill swarms and the emplacement of the Bushveld Complex in the Kaapvaal Craton at about 2050 Ma, the latter indicating increased extensional tectonism, and incipient continental rifting. An overall intracratonic lacustrine tectonic setting for the Pretoria Group is supported by periods of subaerial volcanic activity and palaeosol formation, rapid sedimentary facies changes, significant arkosic sandstones, the presence of non-glacial varves and a highly variable mudrock geochemistry.

  20. Geologic and Geochronologic Studies of the Early Proterozoic Kanektok Metamorphic Complex of Southwestern Alaska

    Turner, Donald L.; Forbes, Robert B.; Aleinikoff, John N.; McDougall, Ian; Hedge, Carl E.; Wilson, Frederic H.; Layer, Paul W.; Hults, Chad P.

    2009-01-01

    The Kanektok complex of southwestern Alaska appears to be a rootless terrane of early Proterozoic sedimentary, volcanic, and intrusive rocks which were metamorphosed to amphibolite and granulite facies and later underwent a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism of overlying sediments. The terrane is structurally complex and exhibits characteristics generally attributed to mantled gneiss domes. U-Th-Pb analyses of zircon and sphene from a core zone granitic orthogneiss indicate that the orthogneiss protolith crystallized about 2.05 b.y. ago and that the protolithic sedimentary, volcanic and granitic intrusive rocks of the core zone were metamorphosed to granulite and amphibolite facies about 1.77 b.y. ago. A Rb-Sr study of 13 whole-rock samples also suggests metamorphism of an early Proterozoic [Paleoproterozoic] protolith at 1.77 Ga, although the data are scattered and difficult to interpret. Seventy-seven conventional 40K/40Ar mineral ages were determined for 58 rocks distributed throughout the outcrop area of the complex. Analysis of the K-Ar data indicate that nearly all of these ages have been totally or partially reset by a pervasive late Mesozoic thermal event accompanied by granitic plutonism and greenschist facies metamorphism. Several biotites gave apparent K-Ar ages over 2 Ga. These ages appear to be controlled by excess radiogenic 40Ar produced by the degassing protolith during the 1.77 Ga metamorphism and incorporated by the biotites when they were at temperatures at which Ar could diffuse through the lattice. Five amphibolites yielded apparent Precambrian 40K/40Ar hornblende ages. There is no evidence that these hornblende ages have been increased by excess argon. The oldest 40K/40Ar hornblende age of 1.77 Ga is identical to the sphene 207Pb/206Pb orthogneiss age and to the Rb-Sr 'isochron' age for six of the 13 whole-rock samples. The younger hornblende ages are interpreted as

  1. Clastic metasediments of the Early Proterozoic Broken Hill Group, New South Wales, Australia: Geochemistry, provenance, and metallogenic significance

    Slack, J.F.; Stevens, B.P.J.

    1994-01-01

    Whole-rock analyses of samples of pelite, psammite, and psammopelite from the Early Proterozoic Broken Hill Group (Willyama Supergroup) in the Broken Hill Block, New South Wales, Australia, reveal distinctive geochemical signatures. Major-element data show high Al2O3 and K2O, low MgO and Na2O, and relatively high Fe2O3T MgO ratios, compared to average Early Proterozoic clastic metasediments. High field strength elements (HFSE) are especially abundant, including Nb (most 15-27 ppm), Ta (most 1.0-2.2 ppm), Th (17-36 ppm), Hf (4-15 ppm), and Zr (most 170-400 ppm); Y (33-74 ppm) is also high. Concentrations of ferromagnesian elements are generally low (Sc = < 20 ppm, Ni = ??? 62 ppm, Co = <26 ppm; Cr = most < 100 ppm). Data for rare earth elements (REEs) show high abundances of light REEs (LaCN = 116-250 ?? chondrite; LaCN = 437 in one sample), high LaCN YbCN ratios (5.6-13.9), and large negative Eu anomalies ( Eu Eu* = 0.32-0.57). The geochemical data indicate derivation of the metasedimentary rocks of the Broken Hill Group by the erosion mainly of felsic igneous (or meta-igneous) rocks. High concentrations of HFSE, Y, and REEs in the metasediments suggest a provenance dominanted by anorogenic granites and(or) rhyolites, including those with A-type chemistry. Likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz + feldspar ?? biotite ?? garnet gneisses (e.g., Potosi-type gneiss) that occur within the lower part of the Willyama Supergroup, or chemically similar basement rocks in the region; alternative sources may have included Early Proterozoic anorogenic granites and(or) rhyolites in the Mount Isa and(or) Pine Creek Blocks of northern Australia, or in the Gawler craton of South Australia. Metallogenic considerations suggest that the metasediments of the Broken Hill Block formed enriched source rocks during the generation of pegmatite-hosted deposits and concentrations of La, Ce, Nb, Ta, Th, and Sn in the region. Li, Be, B, W

  2. A review of the sedimentology of the Early Proterozoic Pretoria Group, Transvaal Sequence, South Africa: implications for tectonic setting

    NASA Astrophysics Data System (ADS)

    Eriksson, P. G.; Schreiber, U. M.; van der Neut, M.

    The sedimentary rocks of the Early Proterozoic Pretoria Group form the floor rocks to teh 2050 M.a. Bushveld Complex. An overall alluvial fan-fan-delta - lacustrine palaeoenvironmental model is postulated for the Pretoria Group. This model is compatible with a continental half-graben tectonic setting, with steep footwall scarps on the southern margin and a lower gradient hanging wall developed to the north. The latter provided much of the basin-fill detritus. It is envisaged that the southern boundary fault system migrated southwards by footwall collapse as sedimentation continued. Synsedimentary mechanical rifting, associated with alluvial and deltaic sedimentation (Rooihoogte-Strubenkop Formations) was followed by thermal subsidence, with concomitant transgressive lacustrine deposition (Daspoort-Magaliesberg Formations). The proposed half-graben basin was probably related to the long-lived Thabazimbi-Murchison and Sugarbush-Barberton lineaments, which bound the preserved outcrops of the Pretoria Group.

  3. The case for simultaneous deformation, metamorphism and plutonism: an example from Proterozoic rocks in central Arizona

    NASA Astrophysics Data System (ADS)

    Karlstrom, K. E.; Williams, M. L.

    1995-01-01

    The syntectonic 1.70 Ga Crazy Basin Monzogranite provides an example of the complex spatial and temporal interactions between metamorphism, deformation, and plutonism. Synchronous plutonism and deformation is indicated by syn-shortening dikes, sills, and veins; parallel magmatic and solid state fabrics; fabrics in xenoliths; and a foliation triple point. Synchronous plutonism and metamorphism is indicated by a systematic increase from 400 °C to 630 °C towards the pluton at a constant pressure of 300 MPa (3 kb). Temperatures are consistent with a conductive cooling model in which a 700 °C pluton was emplaced into country rocks undergoing greenschist facies regional metamorphism. Synchronous deformation and metamorphism is indicated by porphyroblast inclusion geometries that document the synmetamorphic development of the S2 cleavage. The pluton was emplaced adjacent to the Shylock shear zone during progressive shortening. Emplacement of granite as NE-trending sheets was facilitated by temporal partitioning of transpressional convergence into strike-slip and dip-slip components. At the scale of the pluton's aureole and on the relatively rapid time scale of 10 3-10 6 y, regional deformation and metamorphism were punctuated by thermal softening and increased diffusion rates. Data suggests that accretion of Proterozoic arcs in Arizona involved diachronous pluton-enhanced deformation and associated high temperature-low pressure regional metamorphism.

  4. Proterozoic and early Cambrian protists: evidence for accelerating evolutionary tempo

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.

    1994-01-01

    In rocks of late Paleoproterozoic and Mesoproterozoic age (ca. 1700-1000 million years ago), probable eukaryotic microfossils are widespread and well preserved, but assemblage and global diversities are low and turnover is slow. Near the Mesoproterozoic-Neoproterozoic boundary (1000 million years ago), red, green, and chromophytic algae diversified; molecular phylogenies suggest that this was part of a broader radiation of "higher" eukaryotic phyla. Observed diversity levels for protistan microfossils increased significantly at this time, as did turnover rates. Coincident with the Cambrian radiation of marine invertebrates, protistan microfossils again doubled in diversity and rates of turnover increased by an order of magnitude. Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates, within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms.

  5. Proterozoic and early Cambrian protists: evidence for accelerating evolutionary tempo.

    PubMed Central

    Knoll, A H

    1994-01-01

    In rocks of late Paleoproterozoic and Mesoproterozoic age (ca. 1700-1000 million years ago), probable eukaryotic microfossils are widespread and well preserved, but assemblage and global diversities are low and turnover is slow. Near the Mesoproterozoic-Neoproterozoic boundary (1000 million years ago), red, green, and chromophytic algae diversified; molecular phylogenies suggest that this was part of a broader radiation of "higher" eukaryotic phyla. Observed diversity levels for protistan microfossils increased significantly at this time, as did turnover rates. Coincident with the Cambrian radiation of marine invertebrates, protistan microfossils again doubled in diversity and rates of turnover increased by an order of magnitude. Evidently, the Cambrian diversification of animals strongly influenced evolutionary rates, within clades already present in marine communities, implying an important role for ecology in fueling a Cambrian explosion that extends across kingdoms. Images PMID:8041692

  6. Paleomagnetism and geochronology of an Early Proterozoic quartz diorite in the southern Wind River Range, Wyoming, USA

    Harlan, S.S.; Geisman, J.W.; Premo, W.R.

    2003-01-01

    We present geochronologic and paleomagnetic data from a north-trending quartz diorite intrusion that cuts Archean metasedimentary and metaigneous rocks of the South Pass Greenstone Belt of the Wyoming craton. The quartz diorite was previously thought to be either Archean or Early Proterozoic (?) in age and is cut by north and northeast-trending Proterozoic diabase dikes of uncertain age, for which we also report paleomagnetic data. New U-Pb analyses of baddeleyite and zircon from the quartz diorite yield a concordia upper intercept age of 2170 ?? 8 Ma (95% confidence). An 40Ar/39Ar amphibole date from the same sample yields a similar apparent age of about 2124 ?? 30 Ma (2??), thus confirming that the intrusion is Early Proterozoic in age and that it has probably not been thermally disturbed since emplacement. A magmatic event at ca. 2.17 Ga has not previously been documented in the Wyoming craton. The quartz diorite and one of the crosscutting diabase dikes yield essentially identical, well-defined characteristic remanent magnetizations. Results from eight sites in the quartz diorite yield an in situ mean direction of north declination and moderate to steep positive inclination (Dec.=355??, Inc.=65??, k=145, ??95=5??) with a paleomagnetic pole at 84??N, 215??E (??m=6??, ??p=7??). Data from other diabase dike sites are inconsistent with the quartz diorite results, but the importance of these results is uncertain because the age of the dikes is not well known. Interpretation of the quartz diorite remanent magnetization is problematic. The in situ direction is similar to expected directions for magnetizations of Late Cretaceous/early Tertiary age. However, there is no compelling evidence to suggest that these rocks were remagnetized during the late Mesozoic or Cenozoic. Assuming this magnetization to be primary, then the in situ paleomagnetic pole is strongly discordant with poles of 2167, 2214, and 2217 Ma from the Canadian Shield, and is consistent with proposed

  7. Lithology, age and structure of early proterozoic greenstone belts, West African shield

    NASA Technical Reports Server (NTRS)

    Attoh, K.

    1986-01-01

    Lithologic and chemical data have been compiled for belts in the Proterozoic terrane. Available stratigraphic information from geologic maps of these areas indicate that a typical sequence is comprised of predominately mafic lava flows (basalt-andesite) at the base, which are overlain by felsic volcanic rocks including pyroclastic rocks and lavas. Lithostratigraphic data indicate the volcanic succession is 6-8 km thick. This is followed by 3-4 km of basaltic lava flows which are locally pillowed, the top of the unit is marked by a distinctive manganese formation (MF) consisting of Mn-Fe rich cherts up to 200 m thick. The youngest volcanic unit consists of mafic tuffs and breccia with a distinctive fragmental texture. Of about 100 chemical analyses reported calc-alkaline rocks constitute 55% and tholeiites 45%. Quartz-normative basalt constitutes 99% of the rock type in the tholeiitic suite. In the calc-alkaline suite, 9% of the analyses is basalt, 45% andesite and the rest is dacite and rhyodacite. The available data lead to the conclusion that the minimum age for the volcanic activity must be between 2200 and 2100 million years. It is significant that Archean ages have not been reported from any of the volcanic belts (1-10).

  8. Properties of the proterozoic geomagnetic field and geological applications of paleomagnetic data from rocks of the North American Midcontinent rift

    NASA Astrophysics Data System (ADS)

    Kulakov, Evgeniy V.

    Rocks of the North American Midcontinent rift (MCR) exposed in the Lake Superior area provide an excellent opportunity to use paleomagnetism as a means of studying the characteristics of the Proterozoic geomagnetic field and the history of the rift itself. Detailed paleomagnetic and paleointensity studies of different rock units associated with the MCR, including the 1108 Ma alkaline Coldwell Complex (Ontario, Canada), the basaltic lava flows of the Geordie Lake (Ontario, Canada) and Silver Mountain (Upper Michigan, USA) that are assumed to be 1107-1108 Ma, the ˜1095 Ma lava flows of the Portage Lake Volcanics (PLV) (Keweenaw Peninsula, Michigan), and the ˜1088 Ma flows of the Lake Shore Traps (LST) (Keweenaw Peninsula, Michigan) are presented. Paleomagnetic data from the Coldwell Complex indicate that the apparent asymmetry of geomagnetic reversal, recorded by many Keweenawan rocks is an artifact due to fast equator-ward motion of the North American plate during the MCR evolution. The Coldwell Complex data support the validity of the geocentric axial dipole assumption for the ˜1.1 Ga. Extrusive rocks exposed on the Keweenaw Peninsula reveal similar to that of the present day geomagnetic field paleosecular variation. Samples from the ˜1088 Ma Lake Shore Traps yielded consistent paleofield values with a mean value of 26.3 +/- 4.7 μT, which corresponds to a virtual dipole moment of 5.9 +/- 1.1 x 10 22 Am2. The mean and range of paleofield values are similar to those of the recent Earth's magnetic field and incompatible with a "Proterozoic dipole low". These results are consistent with a modern type compositionally-driven geodynamo operating by the end of Mesoproterozoic. New high-quality paleomagnetic poles calculated for the ˜1108 Ma Coldwell Complex and coeval extrusive rocks, and ˜ 1094 Ma PLV indicate that North America was moving directly equator-ward with an approximately 20-25 cm/year rate between 1108 and 1094 Ma with a significant slowdown in motion

  9. Alternative marine and fluvial models for the non-fossiliferous quartzitic sandstones of the Early Proterozoic Daspoort Formation, Transvaal Sequence of southern Africa

    NASA Astrophysics Data System (ADS)

    Eriksson, P. G.; Schreiber, U. M.; van der Neut, M.; Labuschagne, H.; Van Der Schyff, W.; Potgieter, G.

    1993-04-01

    This paper discusses some of the problems related to the palaeoenvironmental interpretation of non-fossiliferous, early Precambrian, recrystallised quartzitic sandstones, using the Early Proterozoic Daspoort Formation, Transvaal Sequence of southern Africa as a case study. These cross-bedded and planar stratified rocks have been interpreted previously as shallow marine deposits, based on limited studies of areas with well-exposed, relatively undeformed outcrops. This postulate rests largely on the apparently mature nature of the recrystallised sandstones and their thin bedding. Examination of outcrops throughout the preserved basin, including those which have been deformed and metamorphosed, reveals the presence of subordinate immature sandstones. Lateral facies relationships permit an alternative distal fan-fluvial braidplain model to be proposed. This is compatible with collected palaeocurrent data, thicknes trends and results of thin section petrography.

  10. The Mackenzie River magnetic anomaly, Yukon and Northwest Territories, Canada-Evidence for Early Proterozoic magmatic arc crust at the edge of the North American craton

    Pilkington, M.; Saltus, R.W.

    2009-01-01

    We characterize the nature of the source of the high-amplitude, long-wavelength, Mackenzie River magnetic anomaly (MRA), Yukon and Northwest Territories, Canada, based on magnetic field data collected at three different altitudes: 300??m, 3.5??km and 400??km. The MRA is the largest amplitude (13??nT) satellite magnetic anomaly over Canada. Within the extent of the MRA, source depth estimates (8-12??km) from Euler deconvolution of low-altitude aeromagnetic data show coincidence with basement depths interpreted from reflection seismic data. Inversion of high-altitude (3.5??km) aeromagnetic data produces an average magnetization of 2.5??A/m within a 15- to 35-km deep layer, a value typical of magmatic arc complexes. Early Proterozoic magmatic arc rocks have been sampled to the southeast of the MRA, within the Fort Simpson magnetic anomaly. The MRA is one of several broad-scale magnetic highs that occur along the inboard margin of the Cordillera in Canada and Alaska, which are coincident with geometric changes in the thrust front transition from the mobile belt to stable cratonic North America. The inferred early Proterozoic magmatic arc complex along the western edge of the North American craton likely influenced later tectonic evolution, by acting as a buttress along the inboard margin of the Cordilleran fold-and-thrust belt. Crown Copyright ?? 2008.

  11. Detrital Zircon Signature of Proterozoic Metasedimentary Rocks of the Pearya Terrane, Northern Ellesmere Island: Implications for Terrane Stratigraphy and Circum-Arctic Terrane Correlations

    NASA Astrophysics Data System (ADS)

    Malone, S. J.; McClelland, W.

    2012-12-01

    The Pearya Terrane, currently recognized as the only exotic terrane in the Canadian Arctic margin, includes early Tonian metaigneous rocks and a sequence of sedimentary rocks ranging from Proterozoic shallow marine to Silurian arc-accretionary units. Succession II (Trettin, 1987) of the Pearya Terrane represents variably metamorphosed metasedimentary rocks of presumed Neoproterozoic to early Ordocician age. These units are structurally juxtaposed with earliest Neoproterozoic orthogneiss of Succession I and the overlaying sedimentary rocks of the Paleozoic section. Detrital zircon age spectra from seven samples of Neoproterozoic meta-sedimentary rocks define three groups on the basis of dominant age peaks and the age of the youngest peaks. Group I, representing three quartzite samples, contains young zircon age peaks at c. 1050 Ma with numerous c. 1100 Ma to 1800 Ma peaks. Detrital zircon spectra from Group I correlate closely with data from the latest Mesoproterozoic Brennevinsfjorden Group of Northeastern Svalbard, suggesting that the base of Succession II may be older than the Succession I orthogneiss, and that the contact between them is tectonic. Group II is defined by a dominant c. 970 Ma age peak that overlaps with ages determined for basement orthogneiss units and indicates that local sedimentary sources, possibly relating to Tonian igneous activity, dominated. Group III displays a similar pattern of c. 1000 Ma to 1800 Ma age peaks to Group I, but contains a small population of c. 600 Ma to 700 Ma grains that are likely sourced from elements of the Timanide orogen and/or the Arctic Alaska-Chukotka (AAC) microplate. The ubiquitous Mesoproterozoic ages suggest extensive sediment input from the Grenville-Svegonorwegian domains of Laurentia and Baltica, either directly or by sediment recycling. This is consistent with detrital zircon datasets from other North Atlantic-Arctic Caledonide terranes, reinforcing stratigraphic links between the Pearya Terrane

  12. Isotopic, geochemical, and temporal characterization of Proterozoic basement rocks in the Quitovac region, northwestern Sonora, Mexico: Implications for the reconstruction of the southwestern margin of Laurentia

    Iriondo, A.; Premo, W.R.; Martínez-Torres, Luis M.; Budahn, J.R.; Atkinson, William W.; Siems, D.F.; Guaras-Gonzalez, B.

    2004-01-01

    A detailed geochemical characterization of 19 representative Proterozoic basement rocks in the Quitovac region in northwestern Sonora, Mexico, has identified two distinct Paleoproterozoic basement blocks that coincide spatially with the previously proposed Caborca and "North America" blocks. New U-Pb zircon geochronology revises their age ranges, the Caborca (1.78-1.69 Ga) and "North America" (1.71-1.66 Ga) blocks at Quitovac, and precludes a simple age differentiation between them. In addition, Grenvillian-age granitoids (ca. 1.1 Ga), spatially associated with the Caborca block have been identified at Quitovac. Nd isotopes and major- and trace-element geochemistry support the distinction of these Paleoproterozoic blocks. Granitoids of the "North America" block are characterized by depleted ??Nd values (3.4-3.9) and younger Nd model ages (1800-1740 Ma) and have lower K2O, Y, Rb, Ba, Th, REE, and Fe/Mg values than coeval rocks of the Caborca block. The Caborca block granitoids are likewise characterized by slightly less depleted ??Nd (0.6-2.6) and older Nd model ages (2070-1880 Ma). Despite the subtle differences, granitoids from both the Caborca and "North America" blocks exhibit island arc-like affinities. We propose that the Proterozoic basement rocks from the Quitovac region are an extension of the Proterozoic crustal provinces in the southwestern United States. Specifically, rocks of the Caborca block exhibit an affinity to rocks of either the Yavapai province or the Mojave-Yavapai transition zone, whereas rocks of the "North America" block have signatures similar to those of the Mazatzal province or possibly the Yavapai province of Arizona. The new isotopic ages and geochemical data do not support the existence of the Late Jurassic Mojave-Sonora megashear at Quitovac, as originally proposed. However, the Quitovac region accounts only for a small fraction of the Proterozoic basement in Sonora, so these findings do not eliminate the possibility of a megashear

  13. The Proterozoic of NW Mexico revisited: U-Pb geochronology and Hf isotopes of Sonoran rocks and their tectonic implications

    NASA Astrophysics Data System (ADS)

    Solari, L. A.; González-León, C. M.; Ortega-Obregón, C.; Valencia-Moreno, M.; Rascón-Heimpel, M. A.

    2018-04-01

    Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U-Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca-Mojave province to the west, characterized by the so-called Bámori Complex, have U-Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved ɛHf values, yielding T DM ages of ca. 2.1-2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile ɛHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile ɛHf values and a T DM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile ɛHf values ranging from -1 to +8 and T DM dates of ca. 1.8-1.9 Ga and 1.6-1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave-Sonora megashear.

  14. Eumetazoan fossils in terminal Proterozoic phosphorites?

    PubMed Central

    Xiao, Shuhai; Yuan, Xunlai; Knoll, Andrew H.

    2000-01-01

    Phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by Ediacaran fossils in terminal Proterozoic sandstones and shales. Phosphorites of the Doushantuo Formation, South China, contain eggs, egg cases, and stereoblastulae that document animals of unspecified phylogenetic position; small fossils containing putative spicules may specifically record the presence of sponges. Microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and/or egg cases modified by diagenetic processes known to have had a pervasive influence on Doushantuo phosphorites. Regardless of this interpretation, evidence for Doushantuo eumetazoans is provided by millimeter-scale tubes that display tabulation and apical budding characteristic of some Cnidaria, especially the extinct tabulates. Like some Ediacaran remains, these small, benthic, colonial fossils may represent stem-group eumetazoans or stem-group cnidarians that lived in the late Proterozoic ocean. PMID:11095754

  15. Eukaryotic organisms in Proterozoic oceans

    PubMed Central

    Knoll, A.H; Javaux, E.J; Hewitt, D; Cohen, P

    2006-01-01

    The geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800–1300 Myr old rocks. 1300–720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran–Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms. PMID:16754612

  16. Early Proterozoic magmatism and tectonism related to southward-dipping subduction and microcontinental accretion in central Wisconsin

    SciT

    Maass, R.S.; Brown, B.A.

    1992-01-01

    A polydeformed and polymetamorphosed terrane of Archean and lower Proterozoic volcanic, plutonic, and sedimentary rocks is exposed in central Wisconsin. The central Wisconsin terrane (CWT) consists primarily of 2,800 and 2,500 Ma gneisses and 1,820-1,890 Ma igneous rocks emplaced into these gneisses during the Penokean orogeny. North of a poorly define northwest-trending suture zone is the 1,8180-1,890 Ma Penokean island-arc terrane of northern Wisconsin, which lacks Archean rocks. Archean and Penokean metamorphism of the CWT each ranged from lower greenschist to upper amphibolite facies. Grade was typically lower to upper amphibolite facies at 2,800 Ma and lower amphibolite facies duringmore » the Penokean orogeny. Locally, a third metamorphic event, possibly 2,500 Ma, has been recognized. The grade of Penokean metamorphism is spatially related to plutons in some areas, but not in others. Most of the CWT underwent one or more phases of deformation during the Penokean orogeny, but at least part of the CWT escaped deformation at this time. A well developed subvertical mineral lineation attributed to diapirism is present in and around many Penokean plutons. The spatial and temporal pattern of igneous activity suggests that the Penokean orogeny involved two simultaneously operating southward-dipping subduction zones. The northern zone produced the island-arc terrane. The southern zone dipped under the CWT microcontinent, producing a continental arc. Petrographic and isotopic data from subsurface samples suggest that the CWT does not extend into southern Wisconsin.« less

  17. Extraterrestrial Impact Episodes and Archaean to Early Proterozoic (3.8 2.4 Ga) Habitats of Life

    NASA Astrophysics Data System (ADS)

    Glikson, Andrew

    The terrestrial record is punctuated by major clustered asteroid and comet impacts, which affected the appearance, episodic extinction, radiation, and reemergence of biogenic habitats. Here I examine manifest and potential extraterrestrial impact effects on the onset and evolution of Archaean to early Proterozoic (3.8- 2.4-Ga) habitats, with reference to the Pilbara (Western Australia) and Kaapvaal (eastern Transvaal) Cratons. The range of extraterrestrial connections of microbial habitats includes cometary contribution of volatiles and amino acids, sterilization by intense asteroid and comet bombardment, supernova and solar flares, and impacttriggered volcanic and hydrothermal activity, tectonic modifications, and tsunami effects. Whereas cometary dusting of planetary atmosphere may contribute littlemodi fied extraterrestrial organic components, large impact effects result in both incineration of organic molecules and shock synthesis of new components. From projected impact incidence, ~1.3% of craters >100 km and ~3.8% of craters >250 km have to date been identified for post-3.8-Ga events, due to the mm-scale of impact spherules and the difficulty in their identification in the field - only the tip of the iceberg is observed regarding the effects of large impacts on the Precambrian biosphere, to date no direct or genetic relations between impacts and the onset or extinction of early Precambrian habitats can be confirmed. However, potential relations include (1) ~3.5-3.43 Ga - intermittent appearance of stromatolite-like structures of possible biogenic origin on felsic volcanic shoals representing intervals between mafic volcanic episodes in rapidly subsiding basins, a period during which asteroid impacts are recorded; (2) ~3.26-3.225 Ga - impact-triggered crustal transformation from mafic-ultramafic volcanic environments to rifted troughs dominated by felsic volcanics and turbidites, marked by a major magmatic peak, resulting in extensive hydrothermal activity and

  18. Provenance and sedimentary environments of the Proterozoic São Roque Group, SE-Brazil: Contributions from petrography, geochemistry and Sm-Nd isotopic systematics of metasedimentary rocks

    NASA Astrophysics Data System (ADS)

    Henrique-Pinto, R.; Janasi, V. A.; Tassinari, C. C. G.; Carvalho, B. B.; Cioffi, C. R.; Stríkis, N. M.

    2015-11-01

    The Proterozoic metasedimentary sequences exposed in the São Roque Domain (Apiaí Terrane, Ribeira Belt, southeast Brazil) consist of metasandstones and meta-felspathic wackes with some volcanic layers of within-plate geochemical signature (Boturuna Formation), a passive margin turbidite sequence of metawackes and metamudstones (Piragibu Formation), and volcano-sedimentary sequences with MORB-like basalts (Serra do Itaberaba Group; Pirapora do Bom Jesus Formation). A combination of zircon provenance studies in metasandstones, whole-rock geochemistry and Sm-Nd isotopic systematics in metamudstones was used to understand the provenance and tectonic significance of these sequences, and their implications to the evolution of the Precambrian crust in the region. Whole-rock geochemistry of metamudstones, dominantly from the Piragibu Formation, points to largely granitic sources (as indicated for instance by LREE-rich moderately fractionated REE patterns and subtle negative Eu anomalies) with some mafic contribution (responding for higher contents of Fe2O3, MgO, V, and Cr) and were subject to moderate weathering (CIA - 51 to 85). Sm-Nd isotope data show three main peaks of Nd TDM ages at ca. 1.9, 2.1 and 2.4 Ga; the younger ages define an upper limit for the deposition of the unit, and reflect greater contributions from sources younger than the >2.1 Ga basement. The coincident age peaks of Nd TDM and U-Pb detrital zircons at 2.1-2.2 Ga and 2.4-2.5 Ga, combined with the possible presence of a small amount of zircons derived from mafic (gabbroid) sources with the same ages, as indicated by a parallel LA-ICPMS U-Pb dating study in metapsammites, are suggestive that these were major periods of crustal growth in the sources involving not only crust recycling but also some juvenile addition. A derivation from similar older Proterozoic sources deposited in a passive margin basin is consistent with the main sedimentary sequences in the São Roque Domain being broadly coeval and

  19. Rocks of the early lunar crust

    NASA Technical Reports Server (NTRS)

    James, O. B.

    1980-01-01

    Data are summarized which suggest a model for the early evolution of the lunar crust. According to the model, during the final stages of accretion, the outer part of the moon melted to form a magma ocean approximately 300 km deep. This ocean fractionated to form mafic and ultramafic cumulates at depth and an overlying anorthositic crust made up of ferroan anorthosites. Subsequent partial melting in the primitive mantle underlying the crystallized magma ocean produced melts which segregated, moved upward, intruded the primordial crust, and crystallized to form layered plutons consisting of Mg-rich plutonic rocks. Intense impact bombardment at the lunar surface mixed and melted the rocks of the two suites to form a thick layer of granulated debris, granulitic breccias, and impact-melt rocks.

  20. Early Proterozoic ties between two suspect terranes and the Mojave crustal block of the Southwestern U.S

    Bender, E. Erik; Morrison, Jean; Anderson, J. Lawford; Wooden, Joseph L.

    1993-01-01

    Southern California and adjacent areas contain two suspect or exotic terranes comprised largely of ancient continental crust, namely the Tujunga (San Gabriel) and Joshua Tree terranes, that have been considered part of a larger displaced terrane, the Santa Lucia-Orocopia allochthon. Paleomagnetic data for the allochthon indicate northward transport in excess of 2000 km and, thus, an origin extraneous to North America. However, Early Proterozoic plutons of the Mojave crustal block and the Joshua Tree and Tujunga terranes have strikingly comparable features, including: (1) crystallization ages of 1.63 to 1.68 Ga; (2) biotite + sphene + magnetite hornblende garnet mineralogy; (3) high LIL and enriched HFS elemental composition; (4) WPG (within-plate granite) trace element chemistry; (5) similar and unique oxygen isotopic compositions; and (6) distinct Pb and Nd isotopic signatures. These features of the Mojave block, which clearly originated as part of native North America, nevertheless distinguish it from crust elsewhere in North America. On the basis of data presented here, we conclude that the Tujunga terrane is a disrupted portion of the Mojave crustal block and is neither far-traveled nor exotic to North America. Its apparent "exotic" nature stems from derivation out of the middle crust. We also conclude that the Joshua Tree terrane is correlative to the Mojave block. We have found no significant evidence for its displacement and consider Joshua Tree to be contiguous with the Mojave block and thus not a valid terrane. The Tujunga (San Gabriel) and Joshua Tree terranes should not be considered as part of, or having shared the same transport as, the Santa Lucia-Orocopia allocthon.

  1. Early Proterozoic (2.04 GA) Phoshorites of Pechenga Greenstone Belt and Their Origin

    NASA Technical Reports Server (NTRS)

    Rozanov, Alexei Yu.; Astafieva, Marina M.; Hoover, Richard B.

    2007-01-01

    No principal differences have been found between microfossils described from Cambrian and Phanerozoic and the 2000 Ma phosphorites. Numerous samples revealed diverse microbial microstructures interpreted as cyanobacterial mats consisting of filamentous (1-3 microns in diameter, 20 microns in length), coccoidal (0.8-1.0 microns) and ellipsoidal or rod-shaped microfossils (0.8 microns in diameter, around 2 microns in length) which morphologically resemble modern Microcoleus and Siphonophycus, Thiocapsa, and Rhabdoderma, respectively, reported from alkali ne or saline environment_ The sequence of the early Palaeoproterozoic events which point to a significant oxidation of the hydrosphere, including the formation of phosphorites and changes in the phosphorous cycle, mimics the sequence which was repeated at the Neoproterozoic-Cembrian transition, implying that oxidation of the terrestrial atmosphere-hydrosphere system experienced an irregular cyclic development.

  2. Microbial mat records in siliciclastic rocks: Examples from Four Indian Proterozoic basins and their modern equivalents in Gulf of Cambay

    NASA Astrophysics Data System (ADS)

    Sarkar, Subir; Banerjee, Santanu; Samanta, Pradip; Chakraborty, Nivedita; Chakraborty, Partha Pratim; Mukhopadhyay, Soumik; Singh, Arvind K.

    2014-09-01

    Microbial mat-related structures (MRS) in siliciclastics have been investigated from four Proterozic formations in India, namely the Marwar Supergroup, the Vindhyan Supergroup, the Chhatisgarh Supergroup and the Khariar Group for their spectral variations, genetic aspects, palaeo-environmental significance and influence on sequence stratigraphic architecture. The maximum diversification of MRS has been experienced in shallow marine coastal Precambrian successions. Observations made from modern environment as well as Precambrian rock records clearly indicates that the features like petee ridges, sand-cracks, gas domes, multi-directed ripples, reticulate surfaces, sieve-like surfaces and setulf are most likely to form in the shallowest part of the marine basins, in upper intertidal to supratidal conditions while wrinkle structures, roll-up structures and patchy ripples had a broader range of palaeogeographic settings from the supratidal to subtidal conditions. Discoidal microbial colony (DMC) represents a special variety of the mat-layer feature in modern environment that may have diverse internal architecture, sometimes falsely resembles Ediacaran medusoids. The uniqueness in sequence stratigraphic architecture of the microbial mat-covered sediment is reflected by the presence of more amalgamated HSTs compare to that of TSTs. The preservation of forced and normal regressive deposits on low-gradient epeiric shelf under low continental freeboard indicates microbial mat-infested sea-floor impedes erosion and concomitant sediment supply may facilitate formation and preservation of regressive packages.

  3. Geochemistry and mineralogy of sediments from the Ventersdorp and Transvaal Supergroups, South Africa: Cratonic evolution during the early Proterozoic

    NASA Astrophysics Data System (ADS)

    Wronkiewicz, David J.; Condie, Kent C.

    1990-02-01

    Approximately 100 pelite and 12 quartzite samples from the Ventersdorp (~2.7 Ga) and Transvaal Supergroups (~2.6-2.1 Ga) have been analyzed to monitor the early Proterozoic evolution of the Kaapvaal Craton, southern Africa. From oldest to youngest, pelites were sampled from the Ventersdorp-Bothaville (BOT), Transvaal-Selati (SEL), Black Reef (BR), Timeball Hill (TH), Strubenkop (STR), and Silverton (SIL) Formations. Paleocurrent measurements in Transvaal quartzites indicate sources lying predominantly to the north and east. Relative to the BOT-SEL-BR, pelites from the TH-STR-SIL are enriched in heavy-REE, LILE, Zr, Hf, Nb, and Ta, depleted in K 2O, MgO, Ni, and Cr, and have lower Cr/Zr, Sc/Th, K 2O/Na 2O, and K/ Rb ratios. Compared to SEL-BR, BOT-TH-STR-SIL pelites have higher light-REE contents and La/Yb ratios, and lower Eu/Eu∗ ratios (0.61-0.66). Relative to NASC (North American Shale Composite), THSTR-SIL pelites are enriched in light-REE, Th, U, Ta, Nb, Sc, Cs, have higher La/Yb ratios, and are depleted in K 2O and MgO. BOT-SEL-BR pelites are enriched in K 2O, MgO, Cr, and Ni, have higher K 2O/Na 2O, Sc/Th, and Eu/Eu∗ ratios, and are depleted in Th, U, heavy-REE, and High Field Strength Elements (HFSE) relative to NASC. Compositions of TH-STR-SIL pelites suggest a provenance similar to average Phanerozoic uppercontinental crust. This source is more evolved than that of BOT-SEL-BR pelites, indicating a transformation from primitive (mafic-rich) to evolved (felsic-rich) upper-crust at 2.2 Ga. This transition follows earlier primitive to evolved trends in Moodies-Pongola (3.3-3.0 Ga) and Witwatersrand (~2.8 Ga) successions. These data suggest that several cycles of changing upper-continental crust occurred in the Kaapvaal craton between 3.3-2.1 Ga.

  4. Exceptional preservation of fossils in an Upper Proterozoic shale

    NASA Technical Reports Server (NTRS)

    Butterfield, N. J.; Knoll, A. H.; Swett, K.

    1988-01-01

    An exceptionally well-preserved and distinctive assemblage of Late Proterozoic fossils from subtidal marine shales is reported. In addition to the spheromorphic acritarchs and cyanobacteria sheaths routinely preserved in Proterozoic rocks, this assemblage includes multicellular algae, a diverse assortment of morphologically complex protistan vesicles, and probable heterotrophic bacteria. Thus, it provides one of the clearest and most taxonomically varied views of Proterozoic life yet reported.

  5. Lead isotope compositions of Late Cretaceous and early Tertiary igneous rocks and sulfide minerals in Arizona: Implications for the sources of plutons and metals in porphyry copper deposits

    Bouse, R.M.; Ruiz, J.; Titley, S.R.; Tosdal, R.M.; Wooden, J.L.

    1999-01-01

    Porphyry copper deposits in Arizona are genetically associated with Late Cretaceous and early Tertiary igneous complexes that consist of older intermediate volcanic rocks and younger intermediate to felsic intrusions. The igneous complexes and their associated porphyry copper deposits were emplaced into an Early Proterozoic basement characterized by different rocks, geologic histories, and isotopic compositions. Lead isotope compositions of the Proterozoic basement rocks define, from northwest to southeast, the Mojave, central Arizona, and southeastern Arizona provinces. Porphyry copper deposits are present in each Pb isotope province. Lead isotope compositions of Late Cretaceous and early Tertiary plutons, together with those of sulfide minerals in porphyry copper deposits and of Proterozoic country rocks, place important constraints on genesis of the magmatic suites and the porphyry copper deposits themselves. The range of age-corrected Pb isotope compositions of plutons in 12 Late Cretaceous and early Tertiary igneous complexes is 206Pb/204Pb = 17.34 to 22.66, 207Pb/204Pb = 15.43 to 15.96, and 208Pb/204Pb = 37.19 to 40.33. These Pb isotope compositions and calculated model Th/U are similar to those of the Proterozoic rocks in which the plutons were emplaced, thereby indicating that Pb in the younger rocks and ore deposits was inherited from the basement rocks and their sources. No Pb isotope differences distinguish Late Cretaceous and early Tertiary igneous complexes that contain large economic porphyry copper deposits from less rich or smaller deposits that have not been considered economic for mining. Lead isotope compositions of Late Cretaceous and early Tertiary plutons and sulfide minerals from 30 metallic mineral districts, furthermore, require that the southeastern Arizona Pb province be divided into two subprovinces. The northern subprovince has generally lower 206Pb/204Pb and higher model Th/U, and the southern subprovince has higher 206Pb/204Pb and

  6. Microbenthic distribution of Proterozoic tidal flats: environmental and taphonomic considerations

    NASA Technical Reports Server (NTRS)

    Kah, L. C.; Knoll, A. H.

    1996-01-01

    Silicified carbonates of the late Mesoproterozoic to early Neoproterozoic Society Cliffs Formation, Baffin Island, contain distinctive microfabrics and microbenthic assemblages whose paleo-environmental distribution within the formation parallels the distribution of these elements through Proterozoic time. In the Society Cliffs Formation, restricted carbonates--including microdigitate stromatolites, laminated tufa, and tufted microbial mats--consist predominantly of synsedimentary cements; these facies and the cyanobacterial fossils they contain are common in Paleoproterozoic successions but rare in Neoproterozoic and younger rocks. Less restricted tidal-flat facies in the formation are composed of laminated microbialites dominated by micritic carbonate lithified early, yet demonstrably after compaction; these strata contain cyanobacteria that are characteristic in Neoproterozoic rocks. Within the formation, the facies-dependent distribution of microbial populations reflects both the style and timing of carbonate deposition because of the strong substrate specificity of benthic cyanobacteria. A reasonable conclusion is that secular changes in microbenthic assemblages through Proterozoic time reflect a decrease in the overall representation of rapidly lithified carbonate substrates in younger peritidal environments, as well as concomitant changes in the taphonomic window of silicification through which early life is observed.

  7. Evidence for Proterozoic and late Cretaceous-early Tertiary ore-forming events in the Coeur d'Alene district, Idaho and Montana

    Leach, D.L.; Hofstra, A.H.; Church, S.E.; Snee, L.W.; Vaughn, R.B.; Zartman, R.E.

    1998-01-01

    New 40Ar/39Ar age spectra on sericite and lead isotope data on tetrahedrite, siderite, galena, bournonite, and stibnite, together with previously published isotopic, geochemical, and geologic studies provide evidence for two major vein-forming events in the Coeur d'Alene district and surrounding area of the Belt basin. The data suggest that the zinc- and lead-rich veins (e.g., Bunker Hill and Star-Morning mines) formed in the Proterozoic (1.0 Ga), whereas the silver-rich veins (e.g., Silver belt mines), antimony veins (e.g., US Antimony mine), and gold-bearing quartz veins (Murry subdistrict) formed in Late Cretaceous to early Tertiary time.

  8. Proterozoic crustal boundary in the southern part of the Illinois Basin

    Heigold, P.C.; Kolata, Dennis R.

    1993-01-01

    Recently acquired COCORP and proprietary seismic reflection data in the southern part of the Illinois Basin, combined with other geological and geophysical data, indicate that a WNW-trending Proterozoic terrane boundary (40 km wide) lies within basement. The boundary is characterized by the termination of subhorizontal Proterozoic reflectors and associated diffraction patterns along a line coinciding with the major magnetic lineament in this region (South Central Magnetic Lineament). North of the boundary, where reflectors thought to represent a sequence of layered Proterozoic rocks in the upper crust are widespread and as much as 11 km thick, total magnetic intensity values are relatively high, suggesting layers of rock with high magnetic susceptibility. To the south, the Proterozoic rocks are acoustically transparent on seismic reflection sections and total magnetic intensity values are relatively low. Moreover, relatively high Bouguer gravity anomaly values to the south may be caused by a dense, altered, lower crustal layer similar to that interpreted from deep seismic refraction studies to underlie the northern Mississippi Embayment. The boundary lies along the projected trend of the northern margin of the Early Proterozoic Central Plains orogen and we suggest that it marks the convergent margin of this orogen. Reactivation of the boundary and the associated zone of weakness during late Paleozoic times apparently resulted in structural deformation in the southern part of the Illinois Basin, including movement along the Cottage Grove Fault System and Ste. Genevieve Fault Zone and igneous activity at Hicks Dome. In addition to the role played by this crustal boundary in the evolution of the Illinois Basin, its location between the Wabash Valley Seismic Zone to the northeast and the New Madrid Seismic Zone to the southwest may be a significant factor in present-day seismicity. ?? 1993.

  9. Two types of ore-bearing mafic complexes of the Early Proterozoic East-Scandinavian LIP and their ore potential

    NASA Astrophysics Data System (ADS)

    Mitrofanov, Felix; Zhirov, Dmitry; Bayanova, Tamara; Korchagin, Alexey; Chaschin, Victor

    2015-04-01

    magma generate single volcano-plutonic rock series. For intrusive ore bodies rock differentiation with the formation of syngenetic wehrlite-clinopyroxenite-gabbro- orthoclase gabbro sequence is typical. Upper mantle source of the depleted magma is characterized by the following isotope indicators: ɛNd(T) +0.5 to +4, ISr= 87Sr/86Sr 0.703-0.704. Ore-bearing intrusive bodies are injected in the upper part of the Early Palaeoproterozoic volcano-sedimentary cross-section. Ores are located in the basement of intrusions and in the redeposited veined bodies, including offset setting. Numerous Ni-Cu deposits with total reserves and resources of several million tons of Nickel equivalent (with an average grade ≥ 0,3%) have been explored, and some of them now is mining. As a result of our research, the complex of indicators and criteria is suggested for predicting the occurrence, for regional exploration target selection and for regional resource evaluation of PGE and base metals. The studies are supported by the Russian Foundation for Basic Research (project nos. 13-05-12055).

  10. Instability of the southern Canadian Shield during the late Proterozoic

    NASA Astrophysics Data System (ADS)

    McDannell, Kalin T.; Zeitler, Peter K.; Schneider, David A.

    2018-05-01

    Cratons are generally considered to comprise lithosphere that has remained tectonically quiescent for billions of years. Direct evidence for stability is mainly founded in the Phanerozoic sedimentary record and low-temperature thermochronology, but for extensive parts of Canada, earlier stability has been inferred due to the lack of an extensive rock record in both time and space. We used 40Ar/39Ar multi-diffusion domain (MDD) analysis of K-feldspar to constrain cratonic thermal histories across an intermediate (∼150-350 °C) temperature range in an attempt to link published high-temperature geochronology that resolves the timing of orogenesis and metamorphism with lower-temperature data suited for upper-crustal burial and unroofing histories. This work is focused on understanding the transition from Archean-Paleoproterozoic crustal growth to later intervals of stability, and how uninterrupted that record is throughout Earth's Proterozoic "Middle Age." Intermediate-temperature thermal histories of cratonic rocks at well-constrained localities within the southern Canadian Shield of North America challenge the stability worldview because our data indicate that these rocks were at elevated temperatures in the Proterozoic. Feldspars from granitic rocks collected at the surface cooled at rates of <0.5 °C/Ma subsequent to orogenesis, seemingly characteristic of cratonic lithosphere, but modeled thermal histories suggest that at ca. 1.1-1.0 Ga these rocks were still near ∼200 °C - signaling either reheating, or prolonged residence at mid-crustal depths assuming a normal cratonic geothermal gradient. After 1.0 Ga, the regions we sampled then underwent further cooling such that they were at or near the surface (≪60 °C) in the early Paleozoic. Explaining mid-crustal residence at 1.0 Ga is challenging. A widespread, prolonged reheating history via burial is not supported by stratigraphic information, however assuming a purely monotonic cooling history requires at the

  11. Introduction: The Proterozoic

    NASA Astrophysics Data System (ADS)

    Jenkins, Gregory S.; McKay, Christopher P.; McMenamin, Mark A. S.

    The Proterozoic (2.5 Ga-545 Ma) is perhaps the most intriguing period in Earth's history. In a typical high school physical science textbook it may be presented as a rather boring period that today's student is happy to pass over in lieu of the Mesozoic and the extinction of Tyrannosaurus rex by a large asteroid. In reality this was a period full of excitement as it opens (in the PalaeoProterozoic) with low-latitude glaciation in concert with a rise in atmospheric oxygen. The Proterozoic ends with a glacial period and a possible rise in atmospheric oxygen levels. Other highlights of the Proterozoic include: three or more severe glacial events, a long period (1 billion years) of apparent warmth without evidence of glacial deposits, significant fluctuations in δC13, two or more periods where supercontinents were assembled, cap carbonates, banded iron formations, the rise of eukaryotes and the first complex life. The juxtaposition of extreme climate conditions and major evolutionary change among complex organisms during the Proterozoic is particularly puzzling, and begs the following question: What are the factors controlling the appearance of complex life?

  12. Re-Os systematics of early proterozoic ferropicrites, Pechenga Complex, northwestern Russia: Evidence for ancient 187Os-enriched plumes

    NASA Astrophysics Data System (ADS)

    Walker, Richard J.; Morgan, John W.; Hanski, Eero J.; Smolkin, Valery F.

    1997-08-01

    The Re-Os isotopic systematics of various ferropicritic flows and sills of the Pechenga Complex, Russia, have been examined. During crystallization about 1.98 Ga ago, many of these bodies became highly differentiated. In addition, some of the larger igneous units are associated with major NiCu ore deposits. The melts that produced these rocks have been termed ferropicritic because of their high FeO and MgO contents. They are also enriched in light rare earth elements (LREEs), TiO 2, Zr, and many other incompatible trace elements. Previous studies have concluded that the ferropicrites were most likely derived from an Fe-rich mantle plume that had a complex history of long-term LREE depletion (initial ɛNd = + 1.4), but that also experienced a LREE enrichment event within 200 Ma of the generation of the rocks. Whole rock samples believed to be most representative of primary melt compositions indicate that initial melt concentrations of rhenium and osmium were approximately 1.1 ppb and 0.5 ppb, respectively. The high primary melt concentrations presumably made the osmium contained in the melts relatively immune to the effects of crustal contamination. Nonetheless, all ore-bearing intrusions examined show osmium isotopic evidence for crustal contamination. For example, the initial γOs for some primary magmatic sulfides from the Pilgujärvi intrusion average +46. Other ore-bearing intrusions, such as the Kammikivi sill, appear to have been similarly contaminated by crustal osmium during the injection of magma, with initial yo, values as high as +251. The seemingly high levels of crustal osmium may be attributed to the rapidly diminishing concentrations of osmium in the melts as the larger bodies differentiated, combined with localized in situ assimilation of the metasedimentary rocks that comprise the country rocks. The Re-Os systematics of some whole rock samples of both mineralized and sulfide-poor intrusions were affected by post-magmatic events, especially the

  13. Genesis of giant Early Proterozoic magnesite and related talc deposits in the Mafeng area, Liaoning Province, NE China

    NASA Astrophysics Data System (ADS)

    Misch, David; Pluch, Hannes; Mali, Heinrich; Ebner, Fritz; Huang, Hui

    2018-07-01

    This study aims to understand the origin of giant magnesite and talc deposits in the Liaohe Group (Liaoning Province, NE China). Magnesite stromatolites and the composition of fluid inclusions suggest that magnesite or high-Mg calcite precipitated directly from strongly restricted seawater pools with meteoric influx. A primary evaporitic origin is also indicated for parts of the investigated dolomites by comparably heavy δ18O values. Later, intense metasomatic activity led to the formation of a magnesite/dolomite succession with irregular contacts and a lighter isotopic signature of oxygen. A slight shift in δ18O to more positive values was observed for talc-hosting magnesite, which can be explained by the incorporation of isotopically light oxygen into talc. This highlights that the hydrothermal processes that led to talc formation influenced the hosting carbonates as well, which is also documented by a tendency to smaller crystal sizes, a higher whiteness and lower trace element concentrations in samples from locations nearby large talc bodies. Although δ13C is suggested to be less sensitive to hydrothermal activity, comparably light δ13C values were determined for magnesite sinters, as well as for remobilized magnesites. In general, the δ13C signature of the investigated magnesites is lighter than expected for Proterozoic carbonates. A single-stage generation of the giant talc deposit in the study area is suggested based on elemental and isotopic data. Later deformation led to a (iso-chemical) re-location of talc at least once. During this process, irregularly distributed, cloudy/massive talc bodies acted as weak zones and were incorporated into shear bands up to several meters in thickness, which form the actually present, structurally controlled deposit. The original ore type is preserved only in areas with minor deformation. Lamprophyre dykes prove Jurassic volcanism and are clearly younger than the main phase of talc generation. A younger (post

  14. Distal alluvial fan sediments in early Proterozoic red beds of the Wilgerivier formation, Waterberg Group, South Africa

    NASA Astrophysics Data System (ADS)

    Van Der Neut, M.; Eriksson, P. G.; Callaghan, C. C.

    The 1900 - 1700 M.a. Waterberg Group belongs to a series of southern African cratonic cover sequences of roughly equivalent age. Red beds of the Wilgerivier Formation comprise sandstones, interbedded with subordinate conglomerates and minor mudrocks. These immature sedimentary rocks exhibit lenticular bedding, radial palaeocurrent patterns and features indicative of both streamflow and gravity-flow deposition. A distal wet alluvial fan palaeoenvironmental setting is envisaged, with fan-deltas forming where alluvial lobes prograded into a lacustrine basin. Intrastratal, diagenetic alteration of ferromagnesian detrital grains and ferruginous grain coatings led to the red colouration of the Wilgerivier sediments.

  15. Carbonate deposition during the late Proterozoic Era: an example from Spitsbergen

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Swett, K.

    1990-01-01

    Carbonate sediments reflect the physico-chemical and biological circumstances of their formation; thus, features of limestones and dolomites may provide insights into both environmental and evolutionary change through geological time. The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale. Although Proterozoic carbonates are often seen as predominantly dolomitic, the Akademikerbreen Group is about 45 percent limestone. Stromatolites are conspicuous in outcrop but constitute only 25 percent of the total section. Micrites and coarser intraclastic carbonates derived mainly from micritric precursors comprise 60 percent of the group, while oolites make up the remaining 15 percent. Distinctive sedimentary features of the group include giant (up to 16 mm) ooids, very early diagenetic calcite nodules and cements, micrites containing subaqueous shrinkage cracks filled with equant microspar cement, and strong 13C enrichment in both carbonates and co-occurring organic matter. The principal features of Akademikerbreen carbonates are widely distributed in coeval successions. However, these rocks appear to differ from older limestones and dolomites in their relative abundance of grainstones and, perhaps, micrites, as well as their paucity of tufa-like laminates and columnar or coniform stromatolites that preserve petrographic evidence of in situ precipitation as a dominant means of carbonate accretion. Upper Proterozoic carbonates also differ from Paleozoic accumulations, but the transition is not abrupt. Most changes accompanying the Proterozoic/Phanerozoic transition can be interpreted in terms of the consequences rather than the causes of metazoan and metaphyte evolution, including the evolution of biomineralization. Carbonate sedimentology reinforces data from other sources which indicate the last 200 to 300 Ma of the Proterozoic Eon was a distinctive interval of

  16. Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans.

    PubMed

    Shen, Bing; Dong, Lin; Xiao, Shuhai; Lang, Xianguo; Huang, Kangjun; Peng, Yongbo; Zhou, Chuanming; Ke, Shan; Liu, Pengju

    2016-01-07

    Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500-750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end.

  17. Molar tooth carbonates and benthic methane fluxes in Proterozoic oceans

    PubMed Central

    Shen, Bing; Dong, Lin; Xiao, Shuhai; Lang, Xianguo; Huang, Kangjun; Peng, Yongbo; Zhou, Chuanming; Ke, Shan; Liu, Pengju

    2016-01-01

    Molar tooth structures are ptygmatically folded and microspar-filled structures common in early- and mid-Proterozoic (∼2,500–750 million years ago, Ma) subtidal successions, but extremely rare in rocks <750 Ma. Here, on the basis of Mg and S isotopes, we show that molar tooth structures may have formed within sediments where microbial sulphate reduction and methanogenesis converged. The convergence was driven by the abundant production of methyl sulphides (dimethyl sulphide and methanethiol) in euxinic or H2S-rich seawaters that were widespread in Proterozoic continental margins. In this convergence zone, methyl sulphides served as a non-competitive substrate supporting methane generation and methanethiol inhibited anaerobic oxidation of methane, resulting in the buildup of CH4, formation of degassing cracks in sediments and an increase in the benthic methane flux from sediments. Precipitation of crack-filling microspar was driven by methanogenesis-related alkalinity accumulation. Deep ocean ventilation and oxygenation around 750 Ma brought molar tooth structures to an end. PMID:26739600

  18. Proterozoic deformation of the East Saharan Craton in Southeast Libya, South Egypt and North Sudan

    NASA Astrophysics Data System (ADS)

    Schandelmeier, H.; Richter, A.; Harms, U.

    1987-09-01

    The basement areas in Southeast Libya, South Egypt and North Sudan, west of the Nile, between Gebel Uweinat and the Bayuda Desert, are part of an approximately 1000-km-wide, complexly folded, polymetamorphic zone with a regional N-NNE-NE-ENE trend of foliation and fold axis. Since this belt extends southwestward into the area of Zalingei in the southern Darfur block (West Sudan), it is named the Northern Zalingei fold zone. Sr and Nd isotopic studies suggest that this zone is older than Pan-African and further indicate that, apart from Archean rocks in the Gebel Uweinat area, this belt is of Early-Middle Proterozoic age. An Early-Middle Proterozoic three-stage deformational and anatectic event established the present-day fold and fault geometry in the western parts of this zone in the Gebel Uweinat—Gebel Kamil area. The Pan-African tectono-thermal episode was most effective in the eastern part of the belt, near the boundary with the Nubian Shield volcano-sedimentary-ophiolite-granitoid assemblages. It caused migmatization, granite emplacement, mylonitization and large-scale wrench faulting which was related to Late Proterozoic accretionary and collisional events of the Arabian-Nubian Shield with the margin of the East Saharan Craton.

  19. Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia: evidence for a major asteroid impact in simatic crustal regions of the early Proterozoic earth

    NASA Astrophysics Data System (ADS)

    Glikson, Andrew; Allen, Charlotte

    2004-04-01

    component to the impact-generated volatile cloud. Conservative mass balance estimates derived from the Ir and Pt flux, assuming global extent of a 10-cm-thick spherule unit and chondritic projectile composition, suggest an asteroid diameter on the scale of ˜30 km. Similar estimates are obtained from spherule sizes, which in DGS4 reach a mean diameter of ˜2.0 mm in aerodynamically elongate spherules. The evidence implies formation of an impact basin on the scale of 400 km in simatic/oceanic regions of the early Proterozoic crust.

  20. The role of biology in planetary evolution: cyanobacterial primary production in low‐oxygen Proterozoic oceans

    PubMed Central

    Bryant, Donald A.; Macalady, Jennifer L.

    2016-01-01

    Summary Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well‐preserved rocks from the Archean (4.0 to 2.5 Gyr ago) and Proterozoic (2.5 to 0.5 Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and membrane‐derived hydrocarbon molecules that are still challenging to interpret. However, it is clear from the sulfur isotope record and other geochemical proxies that the production of oxygen or oxidizing power radically changed Earth's surface and atmosphere during the Proterozoic Eon, pushing it away from the more reducing conditions prevalent during the Archean. In addition to ancient rocks, our reconstruction of Earth's redox evolution is informed by our knowledge of biogeochemical cycles catalysed by extant biota. The emergence of oxygenic photosynthesis in ancient cyanobacteria represents one of the most impressive microbial innovations in Earth's history, and oxygenic photosynthesis is the largest source of O 2 in the atmosphere today. Thus the study of microbial metabolisms and evolution provides an important link between extant biota and the clues from the geologic record. Here, we consider the physiology of cyanobacteria (the only microorganisms capable of oxygenic photosynthesis), their co‐occurrence with anoxygenic phototrophs in a variety of environments and their persistence in low‐oxygen environments, including in water columns as well as mats, throughout much of Earth's history. We examine insights gained from both the rock record and cyanobacteria presently living in early Earth analogue ecosystems and synthesize current knowledge of these ancient microbial mediators in planetary redox evolution. Our analysis supports the hypothesis that anoxygenic

  1. Isotopic and chemical studies of early crustal metasedimentary rocks

    NASA Technical Reports Server (NTRS)

    Jacobsen, Stein B.

    1988-01-01

    The aim, within the bounds of the Early Crustal Genesis Project, was the isotopic and chemical study of selected early crustal meta-sedimentary rocks. Western Australia was chosen as the first field area to examine, as the Yilgarn and Pilbara Blocks comprise one of the largest and most varied Precambrian terranes. Furthermore, the Western Gneiss Terrane (on the western flank of the Yilgarn Block) and the Pilbara Block are both non-greenstone in character; these types of terrane were relatively neglected, but are of great significance in the understanding of early crustal meta-sediments. The meta-sediments of aluminous or peraluminous character, commonly also enriched in Mg and/or Fe relative to the more common pelitic meta-sediments, and at many locations, deficient in one or more of the elements Ca, N, and K, were initially chosen.

  2. Pristine Igneous Rocks and the Early Differentiation of Planetary Materials

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.

    1998-01-01

    Our studies are highly interdisciplinary, but are focused on the processes and products of early planetary and asteroidal differentiation, especially the genesis of the ancient lunar crust. Most of the accessible lunar crust consists of materials hybridized by impact-mixing. Rare pristine (unmixed) samples reflect the original genetic diversity of the early crust. We studied the relative importance of internally generated melt (including the putative magma ocean) versus large impact melts in early lunar magmatism, through both sample analysis and physical modeling. Other topics under investigation included: lunar and SNC (martian?) meteorites; igneous meteorites in general; impact breccias, especially metal-rich Apollo samples and polymict eucrites; effects of regolith/megaregolith insulation on thermal evolution and geochronology; and planetary bulk compositions and origins. We investigated the theoretical petrology of impact melts, especially those formed in large masses, such as the unejected parts of the melts of the largest lunar and terrestrial impact basins. We developed constraints on several key effects that variations in melting/displacement ratio (a strong function of both crater size and planetary g) have on impact melt petrology. Modeling results indicate that the impact melt-derived rock in the sampled, megaregolith part of the Moon is probably material that was ejected from deeper average levels than the non-impact-melted material (fragmental breccias and unbrecciated pristine rocks). In the largest lunar impacts, most of the impact melt is of mantle origin and avoids ejection from the crater, while most of the crust, and virtually all of the impact-melted crust, in the area of the crater is ejected. We investigated numerous extraordinary meteorites and Apollo rocks, emphasizing pristine rocks, siderophile and volatile trace elements, and the identification of primary partial melts, as opposed to partial cumulates. Apollo 15 sample 15434,28 is an

  3. Late Cretaceous remagnetization of Proterozoic mafic dikes, southern Highland Mountains, southwestern Montana: A paleomagnetic and 40Ar/39Ar study

    Harlan, S.S.; Geissman, J.W.; Snee, L.W.; Reynolds, R.L.

    1996-01-01

    Paleomagnetic results from Early Proterozoic metabasite sills and Middle Proterozoic diabase dikes from the southern Highland Mountains of southwestern Montana give well-defined, dual-polarity magnetizations that are statistically identical to those from a small Late Cretaceous pluton that cuts the dikes. The concordance of paleomagnetic directions from rocks of three widely separated ages indicates that the Proterozoic rocks were remagnetized, probably during Late Cretaceous time. Paleomagnetic, rock magnetic, and petrographic observations from the metabasite and diabase samples indicate that remanence is carried primarily by low-Ti magnetite. Combining virtual geomagnetic poles from metabasite sills, diabase dikes, and the Late Cretaceous pluton, we obtain a paleomagnetic pole at 85.5??N, 310.7??E (K = 19.9, A95 = 9.1??, N = 14 sites) that is similar to a reference pole from the 74 Ma Adel Mountain Volcanics of western Montana. Biotite and hornblende 40Ar/39Ar isotopic dates from host basement geneiss and a hornblende from a remagnetized metabasite sill yield ages of ca. 1800 Ma; these dates probably record cooling of the southern Highland Mountains following high-grade metamorphism at 1.9-1.8 Ga. The gneiss and metabasite age spectra show virtually no evidence of disturbance, indicating that the basement rocks were never heated to temperatures sufficient to cause even partial resetting of their argon systems. Thus, the overprint magnetization of the Highland Mountains rocks is not a thermoremanent magnetization acquired during conductive cooling of nearby Late Cretaceous plutons. Remagnetization of the metabasite sills and diabase dikes was probably caused by localized thermochemical and thermoviscous effects during circulation of Late Cretaceous hydrothermal fluids related to epithermal mineralization. The absence of significant disturbance to the 40Ar/39Ar age spectrum from the remagnetized metabasite hornblende indicates that some secondary magnetizations may

  4. Earliest Phanerozoic or latest Proterozoic fossils from the Arabian Shield

    Cloud, P.; Awramik, S.M.; Morrison, K.; Hadley, D.G.

    1979-01-01

    We report here the first biologically definable fossils from pre-Saq (pre-Middle Cambrian) rocks of the Arabian Shield. They include the distinctive helically coiled tubular filaments of the oscillatorialean blue-green alga Obruchevella parva as well as two size classes of spheroidal unicells of uncertain affinity. Also present is the conical stromatolite Conophyton and unidentified stromatolites. All occur in cherty limestones of the Jubaylah Group, northern Saudi Arabia, a nonmarine to locally marine taphrogeosynclinal sequence that fills depressions along the northwest-trending Najd faults. Conophyton has heretofore been found only in strata older than about 680 Ma (except for puzzling records in modern hot springs) while Obruchevella is so far known only from rocks between about 680 and 470 Ma old. Thus it appears that the Jubaylah Group is close to the Proterozoic-Phanerozoic transition. The simple spheroidal nanno-fossils are not diagnostic as to age. Their relationships within what appears to be early diagenetic chert suggest a classical algal-mat association. The brecciated and microchanneled appearance of much of the fossiliferous rock, its locally dolomitic nature, and the prevalence of cryptalgalaminate favors a very shallow, locally turbulent, and perhaps episodically exposed marine or marginal marine setting. The Jubaylah Group lies unconformably beneath the Siq Sandstone (basal member of the Saq Sandstone) of medial Cambrian age, rests nonconformably on crystalline basement, and has yielded a K-Ar whole-rock age (on andesitic basalt) of ???540 Ma. To judge from the fossils, however, that age may be as much as 100 Ma or more too young. ?? 1979.

  5. Neodymium, strontium, and oxygen isotopic variations in the crust of the western United States: Origin of Proterozoic continental crust and tectonic implications

    SciT

    Bennett, V.C.

    1989-01-01

    Initial Nd isotopic ratios of crystalline rocks from an area of about 1.5 {times} 10{sup 6} km{sup 2} of the western United States have been determined in order to map Precambrian age province boundaries and thus document the growth and modification of the North American continent in the Proterozoic. Three age provinces have been delineated. It is demonstrated that large regions of Early Proterozoic continental crust were formed with anomalous isotopic compositions ({sup 143}Nd/{sup 144}Nd ratios lower than Early Proterozoic depleted-mantle). The variations in the initial {epsilon}{sub Nd} and {delta}{sup 18}O values correlate with each other, and correspond to themore » previously determined Nd isotopic provinces. The Pelona, Rand, Chocolate Mountain and Orocopia Schists are represented by 15 lithologically and structurally similar schist bodies exposed along the San Andreas and Garlock faults in southern California. The grayschists have measured {epsilon}{sub Nd} values from -1.7 to -11.7 with depleted-mantle model ages of 0.9 to 1.7 Ga. The Nd isotopic compositions can be modeled as variable mixtures of Early Proterozoic continental crust with a Mesozoic are component. The measured {sup 87}Sr/{sup 86}Sr ratios are from 0.7087 to 0.7129 and reflect the presence of an old continental source. Independent of age, the high initial {epsilon}{sub Nd} values ({sup +}9 {plus minus} 1.5) are consistent with derivation at an oceanic spreading center, either at a MORB or in a back-arc basin environment. The presence of both Early Proterozoic continental detritus and a younger sedimentary component in the grayschist protolith, and the MORB affinity of the metabasalts are compatible with formation of the protoliths of the Pelona and related schists in a Mesozoic basin adjacent to the southwestern United States continental margin.« less

  6. Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

    Patel, S.C.; Frost, B.R.; Chamberlain, K.R.; Snyder, G.L.

    1999-01-01

    The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U-Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U-Pb apatite ages are 2.4-2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz-biotite-plagioclase-garnet-staurolite-kyanite in the pelitic schists; (b) quartz-biotite-plagioclase-low-Ca amphiboles-kyanite in Mg-Al-rich schists, and locally (c) hornblende-plagioclase-garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg-Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from > 7 kbar to < 3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78-1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.

  7. The Source of Proterozoic Anorthosites: Bringing It All Back Home

    NASA Astrophysics Data System (ADS)

    Scoates, J. S.

    2004-05-01

    Proterozoic anorthosites are coarse-grained cumulate igneous rocks dominated by plagioclase of intermediate composition (An70-35) that occur in spatial and temporal association with both intrusions of troctolite and Fe-enriched rocks (ferrodiorite, monzonite) and with predominantly crustally-derived granitic batholiths. Given the relatively limited range of plagioclase compositions within individual intrusions, differences in plagioclase anorthite content between intrusions likely reflects primarily differences in pressures of segregation of plagioclase-rich magma bodies (An content of plag decreases with increasing pressures of crystallization). More importantly, Proterozoic anorthosite plutonic suites formed over an extended interval of time (1.2 byr) during the Middle Proterozoic from 2.1-0.9 Ga and thus are recording fundamental relationships between plate tectonics, mantle temperatures, and crust-mantle interactions over 1/4 of Earth history. Experimental work on opx-normative gabbroic/dioritic rocks from Harp Lake and Rogaland appears to show that some proposed anorthosite parental liquids lie across the trace of the plag+2-px cotectic from 1-1.3 GPa and that they straddle the thermal divide on the plag+px liquidus surface, thus apparently requiring a mafic source region (i.e. lower continental crust). It is unlikely that small amounts of dry partial melting of lower crustal granulite will produce melt compositions that are strongly plag-saturated nor will it yield the large quantities of melt (and corresponding cumulates) required by mass balance constraints. In addition, noritic-gabbronoritic lower crust is opx-normative and cannot be responsible for producing the olivine-bearing anorthosites or troctolites typical of the largest Proterozoic anorthosites. A compilation of high-Al,Fe basaltic magmas from Proterozoic anorthosite plutonic suites worldwide shows them to have compositions that are significantly less silica-rich than the opx-normative rocks that

  8. Heat production in granitic rocks: Global analysis based on a new data compilation

    NASA Astrophysics Data System (ADS)

    Artemieva, I. M.; Thybo, H.; Jakobsen, K.; Sørensen, N. K.; Nielsen, L. S. K.

    2017-12-01

    Granitic rocks play special role in the evolution of the Earth and its thermal regime. Their compositional variability provides constraints on global differentiation processes and large scale planetary evolution, while heat production by radioactive decay is among the main heat sources in the Earth. We analyze a new global database GRANITE2017 on the abundances of Th, U, K and heat production in granitic rocks based on all available published data. Statistical analysis of the data shows a huge scatter in all parameters, but the following conclusions can be made. (i) Bulk heat production in granitic rocks of all ages is ca. 2.0 microW/m3 . It is very low in Archean-Early Proterozoic granitic rocks and there is a remarkable peak in Middle Proterozoic granites followed by a gradual decrease towards Cenozoic granites. (ii) There is no systematic correlation between the tectonically controlled granite-type and bulk heat production, although A-type (anorogenic) granites are the most radioactive, and many of them were emplaced in Middle Proterozoic. (iii) There is no systematic correlation between heat flow and concentrations of radiogenic elements. (iv) The present-day global average Th/U value is 4.75 with a maximum in Archean-Early Proterozoic granites (5.75) and a minimum in Middle-Late Proterozoic granites (3.78). The Th/U ratio at the time of granite emplacement has a minimum in Archean (2.78). (v) The present-day K/U ratio is close to a global estimate for the continental crust only for the entire dataset (1460), but differs from the global ratio for each geological time. (vi) We recognize a sharp change in radiogenic concentrations and ratios from the Early Proterozoic to Middle Proterozoic granites. The Proterozoic anomaly may be caused by major plate reorganizations possibly related to the supercontinent cycle when changes in the granite forming processes may be expected, or it may even indicate a change in global thermal regime, mantle dynamics and plate

  9. Latest Proterozoic stratigraphy and Earth history.

    PubMed

    Knoll, A H; Walter, M R

    1992-04-23

    The end of the Proterozoic Eon was a time of pronounced biological, biogeochemical, climatic and tectonic change. New bio- and chemostratigraphic data provide an improved framework for stratigraphic correlation, making possible a deeper understanding of latest Proterozoic Earth history and providing tools for a chronostratigraphic division of late Proterozoic time.

  10. Rocks Here Sequester Some of Mars Early Atmosphere

    2015-09-02

    This view combines information from two instruments on NASA's Mars Reconnaissance Orbiter to map color-coded composition over the shape of the ground in a small portion of the Nili Fossae plains region of Mars' northern hemisphere. This site is part of the largest known carbonate-rich deposit on Mars. In the color coding used for this map, green indicates a carbonate-rich composition, brown indicates olivine-rich sands, and purple indicates basaltic composition. Carbon dioxide from the atmosphere on early Mars reacted with surface rocks to form carbonate, thinning the atmosphere by sequestering the carbon in the rocks. An analysis of the amount of carbon contained in Nili Fossae plains estimated the total at no more than twice the amount of carbon in the modern atmosphere of Mars, which is mostly carbon dioxide. That is much more than in all other known carbonate on Mars, but far short of enough to explain how Mars could have had a thick enough atmosphere to keep surface water from freezing during a period when rivers were cutting extensive valley networks on the Red Planet. Other possible explanations for the change from an era with rivers to dry modern Mars are being investigated. This image covers an area approximately 1.4 miles (2.3 kilometers) wide. A scale bar indicates 500 meters (1,640 feet). The full extent of the carbonate-containing deposit in the region is at least as large as Delaware and perhaps as large as Arizona. The color coding is from data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), in observation FRT0000C968 made on Sept. 19, 2008. The base map showing land shapes is from the High Resolution Imaging Science Experiment (HiRISE) camera. It is one product from HiRISE observation ESP_010351_2020, made July 20, 2013. http://photojournal.jpl.nasa.gov/catalog/PIA19817

  11. Workshop on Pristine Highlands Rocks and the early History of the Moon

    NASA Technical Reports Server (NTRS)

    Longhi, J. (Editor); Ryder, G. (Editor)

    1983-01-01

    Oxide composition of the Moon, evidence for an initially totally molten Moon, geophysical contraints on lunar composition, random sampling of a layered intrusion, lunar highland rocks, early evolution of the Moon, mineralogy and petrology of the pristine rocks, relationship of the pristine nonmore rocks to the highlands soils and breccias, ferroan anorthositic norite, early lunar igneous history, compositional variation in ferroan anosthosites, a lunar magma ocean, deposits of lunar pristine rocks, lunar and planetary compositions and early fractionation in the solar nebula, Moon composition models, petrogenesis in a Moon with a chondritic refractory lithophile pattern, a terrestrial analog of lunar ilmenite bearing camulates, and the lunar magma ocean are summarized.

  12. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation

    NASA Astrophysics Data System (ADS)

    Sperling, Erik A.; Wolock, Charles J.; Morgan, Alex S.; Gill, Benjamin C.; Kunzmann, Marcus; Halverson, Galen P.; MacDonald, Francis A.; Knoll, Andrew H.; Johnston, David T.

    2015-07-01

    Sedimentary rocks deposited across the Proterozoic-Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities.

  13. Carbonate petrography, kerogen distribution, and carbon and oxygen isotope variations in an early Proterozoic transition from limestone to iron-formation deposition, Transvaal Supergroup, South Africa

    NASA Technical Reports Server (NTRS)

    Beukes, N. J.; Klein, C.; Kaufman, A. J.; Hayes, J. M.

    1990-01-01

    The transition zone comprises Campbellrand microbialaminated (replacing "cryptalgalaminate") limestone and shale, with minor dolomite, conformably overlain by the Kuruman Iron Formation of which the basal part is characterized by siderite-rich microbanded iron-formation with minor magnetite and some hematite-containing units. The iron-formation contains subordinate intraclastic and microbialaminated siderite mesobands and was deposited in deeper water than the limestones. The sequence is virtually unaltered with diagenetic mineral assemblages reflecting a temperature interval of about 110 degrees to 170 degrees C and pressures of 2 kbars. Carbonate minerals in the different rock types are represented by primary micritic precipitates (now recrystallized to microsparite), early precompactional sparry cements and concretions, deep burial limpid euhedral sparites, and spar cements precipitated from metamorphic fluids in close contact with diabase sills. Paragenetic pathways of the carbonate minerals are broadly similar in all lithofacies with kerogen intimately associated with them. Kerogen occurs as pigmentation in carbonate crystals, as reworked organic detritus in clastic-textured carbonate units, and as segregations of kerogen pigment around late diagenetic carbonate crystals. Locally kerogen may also be replaced by carbonate spar. Carbon isotope compositions of the carbonate minerals and kerogen are dependent on their mode of occurrence and on the composition of the dominant carbonate species in a specific lithofacies. Integration of sedimentary, petrographic, geochemical, and isotopic results makes it possible to distinguish between depositional, early diagenetic, deep burial, and metamorphic effects on the isotopic compositions of the carbonate minerals and the kerogen in the sequence. Major conclusions are that deep burial thermal decarboxylation led to 13C depletion in euhedral ferroan sparites and 13C enrichment in kerogen (organic carbon). Metamorphic

  14. Clastic rocks associated with the Midcontinent rift system in Iowa

    Anderson, Raymond R.; McKay, Robert M.

    1997-01-01

    The Middle Proterozoic Midcontinent Rift System (MRS) of North America is a failed rift that formed in response to region-wide stresses about 1,100 Ma. In Iowa, the MRS is buried beneath 2,200?3,500 ft of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of sediments was deposited within basins associated with the rift at several stages during its development. Although the uplift of a rift-axial horst resulted in the erosional removal of most of these clastic rocks from the central region of the MRS in Iowa, thick sequences are preserved in a series of horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously interpreted as indicating the existence of early Paleozoic basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drillhole and were out of stratigraphic position. No deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters useful in differentiating the Proterozoic MRS Red clastics from Paleozoic clastic rocks having similar lithologies.

  15. Highly alkaline lavas in a Proterozoic rift zone: Implications for Precambrian mantle metasomatic processes

    NASA Astrophysics Data System (ADS)

    Gaonac'h, H.; Ludden, J. N.; Picard, C.; Francis, D.

    1992-03-01

    An occurrence of Proterozoic nephelinite and basanite lavas and pyroclastic rocks and associated phonolites indicates that the processes that generate modern alkaline magmas in intraplate settings were operative in the Early Proterozoic. These lavas occur near the top of a 1.9 Ga continental-margin sequence in the Cape Smith fold and thrust belt of northern Quebec. The lavas are classified as nephelinites, basanites, and phonolites on the basis of high field strength and rare earth element contents, although large ion lithophile elements, including alkalis, appear to have been strongly depleted by greenschist facies metamorphism and alteration. Certain major elements define trends consistent with low-pressure fractionation dominated by clinopyroxene, which is the only mafic phenocryst present in the lavas. The mafic and felsic lavas have identical 143Nd/144Nd ratios, consistent with consanguinity and a lack of contamination by older crust of the Superior province. Values for ɛNd (1.96 Ga) of +2 represent an enriched mantle source relative to +4 to +5 for the contemporaneous mid-oceanic-ridge basalt reservoir.

  16. Rocks.

    ERIC Educational Resources Information Center

    Lee, Alice

    This science unit is designed for limited- and non-English speaking students in a Chinese bilingual education program. The unit covers rock material, classification, characteristics of types of rocks, and rock cycles. It is written in Chinese and simple English. At the end of the unit there is a list of main terms in both English and Chinese, and…

  17. Early paleozoic granodioritic plutons in the Shedong W-Mo ore district, Guangxi, southern China: Products of re-melting of middle Proterozoic crust due to magma underplating

    NASA Astrophysics Data System (ADS)

    Jiang, Xingzhou; Kang, Zhiqiang; Xu, Jifeng; Feng, Zuohai; Pang, Chongjin; Fang, Guicong; Wu, Jiachang; Xiong, Songquan

    2017-06-01

    The Shedong W-Mo ore district in the south-central Dayaoshan Uplift of Guangxi, southern China hosts the Baoshan and Pingtoubei deposits, both of which occur in granodioritic plutons. Zircon U-Pb dating of granodiorites and its mafic microgranular enclaves (MMEs) in the Baoshan deposit yielded ages of 439.8 ± 3.2 and 441.1 ± 2.2 Ma, respectively. Granodiorites have moderate SiO2 (54.5-63.0 wt.%) and high Al2O3 (15.4-17.8 wt.%) contents, wide variations in major element ratios, significant rare earth element fractionation, and small negative Eu anomalies. They are rich in Th, U, Zr, and Hf, and depleted in Ba, Nb, and Ti. Their initial 87Sr/86Sr, εNd(t), and εHf(t) values are in the range of 0.7086-0.7091, -5.2 to -6.6 and -6.3 to +1.6, respectively. Rounded or lenticular MMEs have relatively low silica and high mafic components, depletion in Eu, Sr, and Zr, and marked negative Eu anomalies. Rb/Sr and Nb/Ta ratios, and εNd(t) and εHf(t) values of the MMEs are higher than those of host granodiorites, indicating a different magmatic source. Zircon U-Pb dating of the unexposed granodiorite porphyry in the Pingtoubei deposit yielded an age of 440.0 ± 1.7 Ma. The granodiorite porphyries have high SiO2 and low K2O, FeOT, and MgO contents, with similar trace element features to the granodiorites at the Baoshan deposit, although the former has small negative Eu anomalies. Its initial 87Sr/86Sr values range from 0.7162 to 0.7173, εNd(t) values from -8.7 to -12.3, and εHf(t) values from -7.8 to +1.3, indicative of a crustal source. Nd and Hf two-stage model ages of the granodiorites, MMEs, and granodiorite porphyries have a narrow range between 1.3 and 2.2 Ga. We propose that the granodiorites and MMEs at the Baoshan deposit were produced through re-melting of middle Proterozoic crust as a result of underplating of mantle-derived magmas in a transitional compression-to-extension tectonic setting. Mantle-derived magmas provided the heat and material for the formation

  18. Bridging Two Worlds: From the Archean to the Proterozoic

    NASA Technical Reports Server (NTRS)

    Schopf, J. William

    2000-01-01

    As now known, the Archean and Proterozoic appear to have been different worlds: the geology (tectonic style, basinal distribution, dominant rock types), atmospheric composition (O2, CO21, CH4), and surface environment (day-length, solar luminosity, ambient temperature) all appear to have changed over time. And virtually all paleobiologic indicators can be interpreted as suggesting there were significant biotic differences as well: (1) Stromatolites older than 2.5 Ga are rare relative to those of the Proterozoic; their biotic components are largely unknown; and the biogenicity of those older than approx. 3.2 Ga has been questioned. (2) Bona fide microfossils older than approx. 2.4 Ga are rare, poorly preserved, and of uncertain biological relations. Gaps of hundreds of millions of years in the known record make it impossible to show that Archean microorganisms are definitely part of the 2.4 Ga-to-present evolutionary continuum. and (3) In rocks older than approx. 2.2 Ga, the sulfur isotopic record is subject to controversy; phylogenetically distinctive bio-markers are unknown; and nearly a score of geologic units contain organic carbon anomalously light isotopically (relative to that of the Proterozoic and Phanerozoic) that may reflect the presence of Archaeans ("Archaebacteria of earlier classifications) but may not (since cellularly preserved Archean-age Archaeans have never been identified).

  19. New paleomagnetic data from Siberia: Non-uniformitarian geomagnetic field around the Proterozoic-Phanerozoic boundary?

    NASA Astrophysics Data System (ADS)

    Pavlov, V.; Shatsillo, A.; Kouznetsov, N.; Gazieva, E.

    2017-12-01

    There is a range of evidence, mainly from sedimentary and volcanic rocks of the Laurentia and Baltica cratons, that argue for the anomalous character of the Ediacaran-Early Cambrian paleomagnetic record. This feature could be linked either to some peculiarities of the paleomagnetic record itself or to some unusual geophysical event that would have taken place around the Proterozoic-Phanerozoic boundary (e.g., true polar wander or nonuniformitarian geomagnetic field behavior). In the latter case, the traces of this event should be observed in Ediacaran-Early Cambrian rocks anywhere there is a possibility to observe a primary paleomagnetic signal. In previous work, we reported results that suggested an anomalous paleomagnetic record in Siberian Ediacaran-Lower Cambrian rocks. Here we present new Siberian data that indicate a very high geomagnetic reversal frequency during this period and the coexistence of two very different paleomagnetic directions. We speculate that these features could be due either to a near-equatorial geomagnetic dipole during the polarity transitions or to alternation between axial and near equatorial dipoles not directly linked with polarity reversals.

  20. The role of biology in planetary evolution: cyanobacterial primary production in low-oxygen Proterozoic oceans.

    PubMed

    Hamilton, Trinity L; Bryant, Donald A; Macalady, Jennifer L

    2016-02-01

    Understanding the role of biology in planetary evolution remains an outstanding challenge to geobiologists. Progress towards unravelling this puzzle for Earth is hindered by the scarcity of well-preserved rocks from the Archean (4.0 to 2.5 Gyr ago) and Proterozoic (2.5 to 0.5 Gyr ago) Eons. In addition, the microscopic life that dominated Earth's biota for most of its history left a poor fossil record, consisting primarily of lithified microbial mats, rare microbial body fossils and membrane-derived hydrocarbon molecules that are still challenging to interpret. However, it is clear from the sulfur isotope record and other geochemical proxies that the production of oxygen or oxidizing power radically changed Earth's surface and atmosphere during the Proterozoic Eon, pushing it away from the more reducing conditions prevalent during the Archean. In addition to ancient rocks, our reconstruction of Earth's redox evolution is informed by our knowledge of biogeochemical cycles catalysed by extant biota. The emergence of oxygenic photosynthesis in ancient cyanobacteria represents one of the most impressive microbial innovations in Earth's history, and oxygenic photosynthesis is the largest source of O2 in the atmosphere today. Thus the study of microbial metabolisms and evolution provides an important link between extant biota and the clues from the geologic record. Here, we consider the physiology of cyanobacteria (the only microorganisms capable of oxygenic photosynthesis), their co-occurrence with anoxygenic phototrophs in a variety of environments and their persistence in low-oxygen environments, including in water columns as well as mats, throughout much of Earth's history. We examine insights gained from both the rock record and cyanobacteria presently living in early Earth analogue ecosystems and synthesize current knowledge of these ancient microbial mediators in planetary redox evolution. Our analysis supports the hypothesis that anoxygenic photosynthesis

  1. Shell structure and distribution of Cloudina, a potential index fossil for the terminal Proterozoic

    NASA Technical Reports Server (NTRS)

    Grant, S. W.; Knoll, A. H. (Principal Investigator)

    1990-01-01

    Cloudina-bearing biosparites and biomicrites in the lower part of the Nama Group, Namibia, contain a wide morphological diversity of shell fragments that can all be attributed to the two named species C. hartmannae and C. riemkeae. The curved to sinuous tubular shells of Cloudina were multi-layered. Each shell layer was 8 to 50 micrometers thick and in the form of a slightly flaring tube with one end open and the other closed. Growth appears to have been periodic with successive shell layers forming within older layers. Each added layer was slightly elevated from the previous layer at the proximal end and was asymmetrically placed within the older layer so that only a portion of the new shell layer was fused to the previous layer. This type of growth left a relatively large unminerialized area between the shell layers which was often partially or fully occluded by early marine cements. The thin shell layers exhibit both plastic and brittle deformation and were likely formed of a rigid CaCO3-impregnated organic-rich material. Often the shell layers are preferentially dolomitized suggesting an original mineralogy of high-magnesian calcite. Both species in the Nama Group formed thickets, or perhaps bioherms, and this sedentary and gregarious habit suggests that Cloudina was probably a filter-feeding metazoan of at least a cnidarian grade of organization. The unusual shell structure of Cloudina gives rise to a characteristic suite of taphonomic and diagenetic features that can be used to identify Cloudina-bearing deposits within the Nama Group and in other terminal Proterozoic deposits around the world. Species of Cloudina occur in limestones from Brazil, Spain, China, and Oman in sequences consistent with a latest Proterozoic age assignment. In addition, supposed lower Cambrian, pre-trilobitic, shelly fossils from northwest Mexico and the White-Inyo Mountains in California and Nevada, including Sinotubulites, Nevadatubulus, and Wyattia, are all either closely related

  2. Latest Proterozoic stratigraphy and earth history

    NASA Technical Reports Server (NTRS)

    Knoll, Andrew H.; Walter, Malcolm R.

    1992-01-01

    Novel biostratigraphic and chemostratigraphic data furnish an improved framework for stratigraphic correlation of the Proterozoic Eon as well as tools for a chronostratigraphic division of the late Proterozoic. It is argued that, in conjunction with geochronometric data, protistan microfossils and isotope geochemistry can furnish a means for an eventual integration of the latest Proterozoic Eon. Attention is given to the emerging methodologies of fossil protists and prokaryotes and of isotopic chemostratigraphy.

  3. Arsenic stress after the Proterozoic glaciations

    NASA Astrophysics Data System (ADS)

    Chi Fru, Ernest; Arvestål, Emma; Callac, Nolwenn; El Albani, Abderrazak; Kilias, Stephanos; Argyraki, Ariadne; Jakobsson, Martin

    2015-12-01

    Protection against arsenic damage in organisms positioned deep in the tree of life points to early evolutionary sensitization. Here, marine sedimentary records reveal a Proterozoic arsenic concentration patterned to glacial-interglacial ages. The low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after ~2.1 billion years ago. A similar intensification of near continental margin sedimentary arsenic levels after the Cryogenian glaciations is also associated with amplified continental weathering. However, interpreted atmospheric oxygen increase at this time, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life.

  4. Arsenic stress after the Proterozoic glaciations.

    PubMed

    Fru, Ernest Chi; Arvestål, Emma; Callac, Nolwenn; El Albani, Abderrazak; Kilias, Stephanos; Argyraki, Ariadne; Jakobsson, Martin

    2015-12-04

    Protection against arsenic damage in organisms positioned deep in the tree of life points to early evolutionary sensitization. Here, marine sedimentary records reveal a Proterozoic arsenic concentration patterned to glacial-interglacial ages. The low glacial and high interglacial sedimentary arsenic concentrations, suggest deteriorating habitable marine conditions may have coincided with atmospheric oxygen decline after ~2.1 billion years ago. A similar intensification of near continental margin sedimentary arsenic levels after the Cryogenian glaciations is also associated with amplified continental weathering. However, interpreted atmospheric oxygen increase at this time, suggests that the marine biosphere had widely adapted to the reorganization of global marine elemental cycles by glaciations. Such a glacially induced biogeochemical bridge would have produced physiologically robust communities that enabled increased oxygenation of the ocean-atmosphere system and the radiation of the complex Ediacaran-Cambrian life.

  5. Uranium-Lead Zircon Ages and Sr, Nd, and Pb Isotope Geochemistry of Selected Plutonic Rocks from Western Idaho

    Unruh, Daniel M.; Lund, Karen; Kuntz, Mel A.; Snee, Lawrence W.

    2008-01-01

    Across the Salmon River suture in western Idaho, where allochthonous Permian to Cretaceous oceanic rocks are juxtaposed against Proterozoic North American rocks, a wide variety of plutonic rocks are exposed. Available data indicate much variation in composition, source, and structural state of these plutons. The plutonic rocks were long described as the western border zone of the Cretaceous Idaho batholith but limited pre-existing age data indicate more complicated origins. Because the affinity and age of the plutonic rocks cannot be reliably determined from field relations, TIMS U-Pb dating in conjunction with Sr, Nd, and Pb isotopic studies of selected plutons across the suture in western Idaho were undertaken. The data indicate three general groups of plutons including (1) those that intruded the island arc terranes during the Triassic and Jurassic, those that intruded near the western edge of oceanic rocks along the suture in the Early Cretaceous, and the plutons of the Idaho batholith that intruded Proterozoic North American rocks in the Late Cretaceous. Plutons that intruded Proterozoic North American rocks commonly include xenocrystic zircons and in several cases, ages could not be determined. The least radiogenic Sr and most radiogenic Nd are found among the Blue Mountains superterrane island arc samples. Suture-zone plutons have isotopic characteristics that span the range between Idaho batholith and island arc samples but mostly follow island arc signatures. Plutons of the Idaho batholith have the most radiogenic initial Pb and Sr ratios and the least radiogenic Nd of the samples analyzed.

  6. Origin and early evolution of photosynthetic eukaryotes in freshwater environments: reinterpreting proterozoic paleobiology and biogeochemical processes in light of trait evolution.

    PubMed

    Blank, Carrine E

    2013-12-01

    Phylogenetic analyses were performed on concatenated data sets of 31 genes and 11,789 unambiguously alignable characters from 37 cyanobacterial and 35 chloroplast genomes. The plastid lineage emerged somewhat early in the cyanobacterial tree, at a time when Cyanobacteria were likely unicellular and restricted to freshwater ecosystems. Using relaxed molecular clocks and 22 age constraints spanning cyanobacterial and eukaryote nodes, the common ancestor to the photosynthetic eukaryotes was predicted to have also inhabited freshwater environments around the time that oxygen appeared in the atmosphere (2.0-2.3 Ga). Early diversifications within each of the three major plastid clades were also inferred to have occurred in freshwater environments, through the late Paleoproterozoic and into the middle Mesoproterozoic. The colonization of marine environments by photosynthetic eukaryotes may not have occurred until after the middle Mesoproterozoic (1.2-1.5 Ga). The evolutionary hypotheses proposed here predict that early photosynthetic eukaryotes may have never experienced the widespread anoxia or euxinia suggested to have characterized marine environments in the Paleoproterozoic to early Mesoproterozoic. It also proposes that earliest acritarchs (1.5-1.7 Ga) may have been produced by freshwater taxa. This study highlights how the early evolution of habitat preference in photosynthetic eukaryotes, along with Cyanobacteria, could have contributed to changing biogeochemical conditions on the early Earth. © 2013 Phycological Society of America.

  7. Limited role for methane in the mid-Proterozoic greenhouse

    PubMed Central

    Olson, Stephanie L.; Reinhard, Christopher T.; Lyons, Timothy W.

    2016-01-01

    Pervasive anoxia in the subsurface ocean during the Proterozoic may have allowed large fluxes of biogenic CH4 to the atmosphere, enhancing the climatic significance of CH4 early in Earth’s history. Indeed, the assumption of elevated pCH4 during the Proterozoic underlies most models for both anomalous climatic stasis during the mid-Proterozoic and extreme climate perturbation during the Neoproterozoic; however, the geologic record cannot directly constrain atmospheric CH4 levels and attendant radiative forcing. Here, we revisit the role of CH4 in Earth’s climate system during Proterozoic time. We use an Earth system model to quantify CH4 fluxes from the marine biosphere and to examine the capacity of biogenic CH4 to compensate for the faint young Sun during the “boring billion” years before the emergence of metazoan life. Our calculations demonstrate that anaerobic oxidation of CH4 coupled to SO42− reduction is a highly effective obstacle to CH4 accumulation in the atmosphere, possibly limiting atmospheric pCH4 to less than 10 ppm by volume for the second half of Earth history regardless of atmospheric pO2. If recent pO2 constraints from Cr isotopes are correct, we predict that reduced UV shielding by O3 should further limit pCH4 to very low levels similar to those seen today. Thus, our model results likely limit the potential climate warming by CH4 for the majority of Earth history—possibly reviving the faint young Sun paradox during Proterozoic time and challenging existing models for the initiation of low-latitude glaciation that depend on the oxidative collapse of a steady-state CH4 greenhouse. PMID:27671638

  8. The global record of local iron geochemical data from Proterozoic through Paleozoic basins

    NASA Astrophysics Data System (ADS)

    Sperling, E. A.; Wolock, C.; Johnston, D. T.; Knoll, A. H.

    2013-12-01

    Iron-based redox proxies represent one of the most mature tools available to sedimentary geochemists. These techniques, which benefit from decades of refinement, are based on the fact that rocks deposited under anoxic conditions tend to be enriched in highly-reactive iron. However, there are myriad local controls on the development of anoxia, and no local section is an exemplar for the global ocean. The global signal must thus be determined using techniques like those developed to solve an analogous problem in paleobiology: the inference of global diversity patterns through time from faunas seen in local stratigraphic sections. Here we analyze a dataset of over 4000 iron speciation measurements (including over 600 de novo analyses) to better understand redox changes from the Proterozoic through the Paleozoic Era. Preliminary database analyses yield interesting observations. We find that although anoxic water columns in the middle Proterozoic were dominantly ferruginous, there was a statistical tendency towards euxinia not seen in early Neoproterozoic or Ediacaran data. Also, we find that in the Neoproterozoic oceans, oxic depositional environments-the likely home for early animals-have exceptionally low pyrite contents, and by inference low levels of porewater sulfide. This runs contrary to notions of sulfide stress on early metazoans. Finally, the current database of iron speciation data does not support an Ediacaran or Cambrian oxygenation event. This conclusion is of course only as sharp as the ability of the Fe-proxy database to track dissolved oxygen and does not rule out the possibility of a small-magnitude change in oxygen. It does suggest, however, that if changing pO2 facilitated animal diversification it did so by a limited rise past critical ecological thresholds, such as seen in the modern Oxygen Minimum Zones benthos. Oxygen increase to modern levels thus becomes a Paleozoic problem, and one in need of better sampling if a database approach is to be

  9. Late Proterozoic diabase dikes of the New Jersey Highlands; a remnant of Iapetan rifting in the north-central Appalachians

    Volkert, R.A.; Puffer, J.H.

    1995-01-01

    Diabase dikes of widespread occurrence intrude only middle Proterozoic rocks in the New Jersey Highlands. These dikes are enriched in TiO2, P2O5, Zr, and light rare earth elements, and have compositions that range from tholeiitic to alkalic. Dike descriptions, field relations, petrography, geochemistry, petrogenesis, and tectonic setting are discussed. The data are consistent with emplacement in a rift-related, within-plate environment and suggest a correlation with other occurrences of late Proterozoic Appalachian basaltic magmatism.

  10. Experimental Study on the Coupling Mechanism of Early-strength Backfill and Rock

    NASA Astrophysics Data System (ADS)

    Wang, Mingxu

    2017-11-01

    In order to study the interaction mechanism between the ore rock and backfill at the early stage, paraffin is chosen as the cementing agent. Based on the damage mechanics and fractal theory, the interaction mechanism between the ore rock and backfill is characterized by the relevant tests on the complex of proportioned ore rock and backfill with resistance strain gauge, crack propagation, microscopic imaging and AE. The experimental results showed that: 1) Through the axial loading test, compared with the early strength of the cemented filling and paraffin mechanical deformation characteristics, the stress and strain curves of the two had a common linear deformation law, while in the early strength of the filling elastic capacity strong, with a certain degree of resilience. 2) The bearing capacity of the backfill was weak, but the deformation ability was strong. During the bearing process, the deformation of the upper load was mainly caused by the ore rock, which leaded to the damage of the rock. 3) The distribution of AE points during the co-carrying of the filling and the ore rock was monitored by the acoustic emission instrument. The damage occurred mainly in the contact zone between the backfill and the ore rock zone. The corresponding AE point distribution also validated the crack happening.

  11. Experimental acidification of Little Rock Lake (Wisconsin): fish research approach and early responses.

    PubMed

    Swenson, W A; McCormick, J H; Simonson, T D; Jensen, K M; Eaton, J G

    1989-01-01

    One goal of research at Little Rock Lake, Wisconsin, is to enhance understanding of lake acidification effects on warm- and cool-water fishery resources. The Little Rock Lake fish assemblage is characteristic of many acid sensitive waters in North America and is dominated by yellow perch (Percidae) and sunfishes (Centrarchidae). Analyses of reproduction, early survival and growth rates in the field were designed around the differing reproductive modes of these taxa. Complementary laboratory research on early life stages was conducted to assist in isolating direct effect mechanisms and to determine the reliability of laboratory results in predicting field response. Preliminary findings suggest that lake acidification to pH 5.6 has not influenced reproductive activity of the four most abundant fish species. However, the field results suggest that year-class failure of rock bass (Ambloplites rupestris) may be occurring due to reduced survival of early life stages. Reduced growth and food conversion efficiency of Age 0 largemouth bass (Micropterus salmoides) is also suggested. The laboratory bioassays indicate rock bass is the most acid-sensitive Little Rock Lake species tested. However, rock bass fry survival was not significantly affected until pH was reduced from 5.6 to 5.0.

  12. Organic tissues, graphite, and hydrocarbons in host rocks of the Rum Jungle Uranium Field, northern Australia

    Foster, C.B.; Robbins, E.I.; Bone, Y.

    1990-01-01

    The Rum Jungle Uranium field consists of at least six early Proterozoic deposits that have been mined either for uranium and/or the associated base and precious metals. Organic matter in the host rocks of the Whites Formation and Coomalie Dolomite is now predominantly graphite, consistent with the metamorphic history of these rocks. For nine samples, the mean total organic carbon content is high (3.9 wt%) and ranged from 0.33 to 10.44 wt%. Palynological extracts from the host rocks include black, filamentous, stellate (Eoastrion-like), and spherical morphotypes, which are typical of early Proterozoic microbiota. The colour, abundance, and shapes of these morphotypes reflect the thermal history, organic richness, and probable lacustrine biofacies of the host rocks. Routine analysis of rock thin sections and of palynological residues shows that mineral grains in some of the host rocks are coated with graphitized organic matter. The grain coating is presumed to result from ultimate thermal degradation of a petroleum phase that existed prior to metamorphism. Hydrocarbons are, however, still present in fluid inclusions within carbonates of the Coomalie Dolomite and lower Whites Formation. The fluid inclusions fluoresce dull orange in blue-light excitation and their hydrocarbon content is confirmed by gas chromatography of whole-rock extracts. Preliminary analysis of the oil suggests that it is migrated, and because it has escaped graphitization through metamorphism it is probably not of early Proterozoic age. The presence of live oil is consistent with fluid inclusion data that suggest subsequent, low-temperature brine migration through the rocks. The present observations support earlier suggestions that organic matter in the host formations trapped uranium to form protore. Subsequent fluid migrations probably brought additional uranium and other metals to these formations, and the organic matter provided a reducing environment for entrapment. ?? 1990.

  13. Pristine Igneous Rocks and the Genesis of Early Planetary Crusts

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.; Lindstrom, David (Technical Monitor)

    2002-01-01

    Our studies are highly interdisciplinary, but are focused on the processes and products of early planetary and asteroidal differentiation, especially the genesis of the ancient lunar crust. The compositional diversity that we explore is the residue of process diversity, which has strong relevance for comparative planetology.

  14. The environmental distribution of late proterozoic organisms

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.

    1991-01-01

    Along present day coast lines, the environmental distributions of prokaryotic and protistan populations are often sharply delimited. Realized habitat ranges are generally narrower than those circumscribed by physiological tolerances, suggesting the importance of organism-organism interactions in the determination of population distributions. Microfossil populations preserved in silicified carbonates, dolomites, and shales of the 700-800 Ma old Akademikerbreen Group, Svalbard, and elsewhere indicate that the environmental distributions were defined equally clearly during the Proterozoic Eon. The Draken Conglomerate Formation is a tidal flat/lagoonal complex in which we have distinguished five principle biofacies containing a total of 42 taxa. Supratidal to subtidal gradients include the increasing abundance and diversity of both mat dweller microbenthos and allochthonous (principally planktonic) organisms, as well as a taphonomically important pattern of decreasing sheath thickness among mat builder microorganisms. The seaward barriers of Akademikerbreen lagoons were oolitic shoals, and these contain about a dozen endolithic and epilithic species not observed elsewhere in the group. Subtidal environments below fair weather wave base are represented by mudstones of the Svanbergfjellet Formation. These contain abundant and diverse cyanobacteria-like fossils generally similar to but specifically different from those found in tidal flat sediments, as well as diverse unicellular protists (some of impressive morphological complexity) and at least half a dozen cellularly preserved metaphyte populations. In all, more than 80 species are distributed among Akademikerbreen lithologies. Fossil assemblages from Svalbard and elsewhere illustrate the potential for a much finer paleoecological, biostratigraphic, and, hence, evolutionary resolution of the early fossil record.

  15. Assembling and disassembling california: A zircon and monazite geochronologic framework for proterozoic crustal evolution in southern California

    Barth, A.P.; Wooden, J.L.; Coleman, D.S.; Vogel, M.B.

    2009-01-01

    The Mojave province in southern California preserves a comparatively complete record of assembly, postorogenic sedimentation, and rifting along the southwestern North American continental margin. The oldest exposed rocks are metasedimentary gneisses and amphibolite, enclosing intrusive suites that range from tonalite and quartz mon-zodiorite to granite with minor trondhjemite. Discrete magmatic episodes occurred at approximately 1790-1730 and 1690-1640 Ma. Evidence from detrital and premagmatic zircons indicates that recycling of 1900-1790 Ma Paleopro-terozoic crust formed the unique isotopic character of the Mojave province. Peak metamorphic conditions in the Mojave province reached middle amphibolite to granulite facies; metamorphism occurred locally from 1795 to 1640 Ma, with widespread evidence for metamorphism at 1711-1689 and 1670-1650 Ma. Structures record early, tight to isoclinal folding and penetrative west-vergent shear during the final metamorphic event in the west Mojave province. Proterozoic basement rocks are overlain by siliciclastic-carbonate sequences of Mesoproterozoic, Neoproterozoic, and Cambrian age, recording environmental change over the course of the transition from stable Mojave crust to the rifted Cordilleran margin. Neoproterozoic quartzites have diverse zircon populations inconsistent with a southwest North American source, which we infer were derived from the western conjugate rift pair within Rodinia, before establishment of the miogeocline. Neoproterozoic-Cambrian miogeoclinal clastic rocks record an end to rifting and establishment of the Cordilleran miogeocline in southern California by latest Neoproterozoic to Early Cambrian time. ?? 2009 by The University of Chicago.

  16. Pristine Igneous Rocks and the Early Differentiation of Planetary Materials

    NASA Technical Reports Server (NTRS)

    Warren, Paul H.

    2005-01-01

    Our studies are highly interdisciplinary, but are focused on the processes and products of early planetary and asteroidal differentiation, especially the genesis of the ancient lunar crust. The compositional diversity that we explore is the residue of process diversity, which has strong relevance for comparative planetology. Most of the accessible lunar crust consists of materials hybridized by impact-mixing. Our lunar research concentrates on the rare pristine (unmixed) samples that reflect the original genetic diversity of the early crust. Among HED basalts (eucrites and clasts in howardites), we distinguish as pristine the small minority that escaped the pervasive thermal metamorphism of the parent asteroid's crust. We have found a correlation between metamorphically pristine HED basalts and the similarly small minority of compositionally evolved "Stannern trend" samples, which are enriched in incompatible elements and titanium compared to main group eucrites, and yet have relatively high mg ratios. Other topics under investigation included: lunar and SNC (martian?) meteorites; igneous meteorites in general; impact breccias, especially metal-rich Apollo samples and polymict eucrites; siderophile compositions of the lunar and martian mantles; and planetary bulk compositions and origins.

  17. Kilbuck terrane: Oldest known rocks in Alaska

    SciT

    Box, S.E.; Moll-Stalcup, E.J.; Wooden, J.L.

    1990-12-01

    The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2,070 {plus minus}16 and 2,040 {plus minus}74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite ({epsilon}{sub Nd}(T) = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton ({epsilon}{sub Nd}(T) = {minus}5.7) containsmore » a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. However, Phanerozoic plutons cutting several continental terranes in Alaska (southern Brooks Range and Ruby, Seward, and Yukon-Tanana terranes) have Nd isotope compositions indicative of Early Proterozoic (or older) crustal components that could be correlative with rocks of the Kilbuck terrane. Rocks with similar igneous ages in cratonal North America are rare, and those few that are known have Nd isotope compositions distinct from those of the Kilbuck terrane. Conversely, provinces with Nd model ages of 2.0-2.1 Ga are characterized by extensive 1.8 Ga or younger plutonism, which is unknown in the Kilbuck terrane. At present the case for a North American parentage of the Kilbuck terrane is not compelling. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded.« less

  18. Seismites in a Proterozoic tidal succession, Singhbhum, Bihar, India

    NASA Astrophysics Data System (ADS)

    Bhattacharya, H. N.; Bandyopadhyay, Sandip

    1998-08-01

    Early Proterozoic metasediments of the Chaibasa Formation (Galudih-Ghatsila-Dhalbhumgarh region, Singhbhum, Bihar, India) comprise a number of cyclic fining-upward prograding successions of tidalites. The tidalites show indications for earthquakes in the form of synsedimentary deformation features, apart from the structures due to high-energy wave action. Deformed cross-bedding, convolute laminations, synsedimentary faults, graben-like structures, sandstone dykes, pseudonodules and slump folds record the seismic activity. A gradual decline in the frequency of seismites and tsunami-related depositional features, in combination with an upward increase in thickness of the tidal cycles, are attributed to gradual diminishing of tectonic activity within the basin.

  19. Reappraisal of hydrocarbon biomarkers in Archean rocks

    PubMed Central

    French, Katherine L.; Hallmann, Christian; Hope, Janet M.; Schoon, Petra L.; Zumberge, J. Alex; Hoshino, Yosuke; Peters, Carl A.; George, Simon C.; Love, Gordon D.; Brocks, Jochen J.; Buick, Roger; Summons, Roger E.

    2015-01-01

    Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (<37.9 pg per gram of rock) and total sterane (<32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories. PMID:25918387

  20. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada

    NASA Astrophysics Data System (ADS)

    Tashiro, Takayuki; Ishida, Akizumi; Hori, Masako; Igisu, Motoko; Koike, Mizuho; Méjean, Pauline; Takahata, Naoto; Sano, Yuji; Komiya, Tsuyoshi

    2017-09-01

    The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible, and biogenic graphite has thus far been found only in the 3.7-3.8 Ga (gigayears ago) Isua supracrustal belt. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δ13Corg values of -28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.

  1. Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada.

    PubMed

    Tashiro, Takayuki; Ishida, Akizumi; Hori, Masako; Igisu, Motoko; Koike, Mizuho; Méjean, Pauline; Takahata, Naoto; Sano, Yuji; Komiya, Tsuyoshi

    2017-09-27

    The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible, and biogenic graphite has thus far been found only in the 3.7-3.8 Ga (gigayears ago) Isua supracrustal belt. Here we present the total organic carbon contents and carbon isotope values of graphite (δ 13 C org ) and carbonate (δ 13 C carb ) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δ 13 C org values of -28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ 13 C org values and metamorphic grade indicates that variations in the δ 13 C org values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ 13 C carb and δ 13 C org values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.

  2. In search of early life: Carbonate veins in Archean metamorphic rocks as potential hosts of biomarkers

    NASA Astrophysics Data System (ADS)

    Peters, Carl A.; Piazolo, Sandra; Webb, Gregory E.; Dutkiewicz, Adriana; George, Simon C.

    2016-11-01

    The detection of early life signatures using hydrocarbon biomarkers in Precambrian rocks struggles with contamination issues, unspecific biomarkers and the lack of suitable sedimentary rocks due to extensive thermal overprints. Importantly, host rocks must not have been exposed to temperatures above 250 °C as at these temperatures biomarkers are destroyed. Here we show that Archean sedimentary rocks from the Jeerinah Formation (2.63 billion yrs) and Carawine Dolomite (2.55 billion yrs) of the Pilbara Craton (Western Australia) drilled by the Agouron Institute in 2012, which previously were suggested to be suitable for biomarker studies, were metamorphosed to the greenschist facies. This is higher than previously reported. Both the mineral assemblages (carbonate, quartz, Fe-chlorite, muscovite, microcline, rutile, and pyrite with absence of illite) and chlorite geothermometry suggest that the rocks were exposed to temperatures higher than 300 °C and probably ∼400 °C, consistent with greenschist-facies metamorphism. This facies leads to the destruction of any biomarkers and explains why the extraction of hydrocarbon biomarkers from pristine drill cores has not been successful. However, we show that the rocks are cut by younger formation-specific carbonate veins containing primary oil-bearing fluid inclusions and solid bitumens. Type 1 veins in the Carawine Dolomite consist of dolomite, quartz and solid bitumen, whereas type 2 veins in the Jeerinah Formation consist of calcite. Within the veins fluid inclusion homogenisation temperatures and calcite twinning geothermometry indicate maximum temperatures of ∼200 °C for type 1 veins and ∼180 °C for type 2 veins. Type 1 veins have typical isotopic values for reprecipitated Archean sea-water carbonates, with δ13CVPDB ranging from - 3 ‰ to 0‰ and δ18OVPDB ranging from - 13 ‰ to - 7 ‰, while type 2 veins have isotopic values that are similar to hydrothermal carbonates, with δ13CVPDB ranging from - 18

  3. Geochronological and lead-isotope evidences for rapid crust formation in middle-proterozoic time: The Labrador example

    NASA Technical Reports Server (NTRS)

    Schaerer, Urs

    1988-01-01

    Extensive U-Pb geochronological studies in the Grenville and Makkovik provinces have shown that eastern Labrador is underlain by two distinct crustal blocks. In order to substantiate the juvenile character of the middle-Proterozoic crustal block, the isotopic compositon of lead in leached k-feldspars from the same rocks were analyzed. The results of the analysis are briefly discussed.

  4. Prokaryotic algae associated with Australian proterozoic stromatolites.

    NASA Technical Reports Server (NTRS)

    Licari, G. R.; Cloud, P.

    1972-01-01

    The most favorable sites in which to study the associations between stromatolites and the algae responsible for them are places where a variety of stromatolites of possibly early diagenetic or primary silica occupy a layer of substantial thickness of little metamorphosed ancient sediments. One such place is in northwestern Queensland, Australia. Five cases of association between stromatolites and blue-green algal nannofossils were observed within a 100-m sequence of carbonate rocks in that area.

  5. Temporal and structural evolution of the Early Palæogene rocks of the Seychelles microcontinent.

    PubMed

    Shellnutt, J Gregory; Yeh, Meng-Wan; Suga, Kenshi; Lee, Tung-Yi; Lee, Hao-Yang; Lin, Te-Hsien

    2017-03-14

    The Early Palæogene Silhouette/North Island volcano-plutonic complex was emplaced during the rifting of the Seychelles microcontinent from western India. The complex is thought to have been emplaced during magnetochron C28n. However, the magnetic polarities of the rocks are almost entirely reversed and inconsistent with a normal polarity. In this study we present new in situ zircon U/Pb geochronology of the different intrusive facies of the Silhouette/North Island complex in order to address the timing of emplacement and the apparent magnetic polarity dichotomy. The rocks from Silhouette yielded weighted mean 206 Pb/ 238 U ages from 62.4 ± 0.9 Ma to 63.1 ± 0.9 Ma whereas the rocks from North Island yielded slightly younger mean ages between 60.6 ± 0.7 Ma to 61.0 ± 0.8 Ma. The secular latitudinal variation from Silhouette to North Island is consistent with the anticlockwise rotation of the Seychelles microcontinent and the measured polarities. The rocks from Silhouette were emplaced across a polarity cycle (C26r-C27n-C27r) and the rocks from North Island were emplaced entirely within a magnetic reversal (C26r). Moreover, the rocks from North Island and those from the conjugate margin of India are contemporaneous and together mark the culmination of rift-related magmatism.

  6. Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Grotzinger, J. P.; Kaufman, A. J.; Kolosov, P.

    1995-01-01

    In the Olenek Uplift of northeastern Siberia, the Khorbusuonka Group and overlying Kessyusa and Erkeket formations preserve a significant record of terminal Proterozoic and basal Cambrian Earth history. A composite section more than 350 m thick is reconstructed from numerous exposures along the Khorbusuonka River. The Khorbusuonka Group comprises three principal sedimentary sequences: peritidal dolomites of the Mastakh Formation, which are bounded above and below by red beds; the Khatyspyt and most of the overlying Turkut formations, which shallow upward from relatively deep-water carbonaceous micrites to cross-bedded dolomitic grainstones and stromatolites; and a thin upper Turkut sequence bounded by karst surfaces. The overlying Kessyusa Formation is bounded above and below by erosional surfaces and contains additional parasequence boundaries internally. Ediacaran metazoans, simple trace fossils, and vendotaenids occur in the Khatyspyt Formation; small shelly fossils, more complex trace fossils, and acritarchs all appear near the base of the Kessyusa Formation and diversify upward. The carbon-isotopic composition of carbonates varies stratigraphically in a pattern comparable to that determined for other terminal Proterozoic and basal Cambrian successions. In concert, litho-, bio-, and chemostratigraphic data indicate the importance of the Khorbusuonka Group in the global correlation of terminal Proterozoic sedimentary rocks. Stratigraphic data and a recently determined radiometric date on basal Kessyusa volcanic breccias further underscore the significance of the Olenek region in investigations of the Proterozoic-cambrian boundary.

  7. Integrated approaches to terminal Proterozoic stratigraphy: an example from the Olenek Uplift, northeastern Siberia.

    PubMed

    Knoll, A H; Grotzinger, J P; Kaufman, A J; Kolosov, P

    1995-01-01

    In the Olenek Uplift of northeastern Siberia, the Khorbusuonka Group and overlying Kessyusa and Erkeket formations preserve a significant record of terminal Proterozoic and basal Cambrian Earth history. A composite section more than 350 m thick is reconstructed from numerous exposures along the Khorbusuonka River. The Khorbusuonka Group comprises three principal sedimentary sequences: peritidal dolomites of the Mastakh Formation, which are bounded above and below by red beds; the Khatyspyt and most of the overlying Turkut formations, which shallow upward from relatively deep-water carbonaceous micrites to cross-bedded dolomitic grainstones and stromatolites; and a thin upper Turkut sequence bounded by karst surfaces. The overlying Kessyusa Formation is bounded above and below by erosional surfaces and contains additional parasequence boundaries internally. Ediacaran metazoans, simple trace fossils, and vendotaenids occur in the Khatyspyt Formation; small shelly fossils, more complex trace fossils, and acritarchs all appear near the base of the Kessyusa Formation and diversify upward. The carbon-isotopic composition of carbonates varies stratigraphically in a pattern comparable to that determined for other terminal Proterozoic and basal Cambrian successions. In concert, litho-, bio-, and chemostratigraphic data indicate the importance of the Khorbusuonka Group in the global correlation of terminal Proterozoic sedimentary rocks. Stratigraphic data and a recently determined radiometric date on basal Kessyusa volcanic breccias further underscore the significance of the Olenek region in investigations of the Proterozoic-cambrian boundary.

  8. AMA0428, A Potent Rock Inhibitor, Attenuates Early and Late Experimental Diabetic Retinopathy.

    PubMed

    Hollanders, Karolien; Hove, Inge Van; Sergeys, Jurgen; Bergen, Tine Van; Lefevere, Evy; Kindt, Nele; Castermans, Karolien; Vandewalle, Evelien; van Pelt, Jos; Moons, Lieve; Stalmans, Ingeborg

    2017-02-01

    Diabetic retinopathy (DR) is characterized by an early stage of inflammation and vessel leakage, and an advanced vasoproliferative stage. Also, neurodegeneration might play an important role in disease pathogenesis. The aim of this study was to investigate the effect of the Rho kinase (ROCK) inhibitor, AMA0428, on these processes. The response to ROCK inhibition by AMA0428 (1 µg) was studied in vivo using the murine model for streptozotocin (STZ)-induced diabetes, focusing on early non-proliferative DR features and the oxygen-induced retinopathy (OIR) model to investigate proliferative DR. Intravitreal (IVT) administration of AMA0428 was compared with murine anti-VEGF-R2 antibody (DC101, 6.2 µg) and placebo (H 2 O/PEG; 1C8). Outcome was assessed by analyzing leukostasis using fluorescein isothiocyanate coupled concanavalin A (FITC-ConA) and vessel leakage (bovine serum albumin conjugated with fluorescein isothiocyanate; FITC-BSA)/neovascularization and neurodegeneration by immunohistological approaches (hematoxylin and eosin (H&E), terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL), Brn3a). ELISA and Western blotting were employed to unravel the consequences of ROCK inhibition (1 µM AMA0428) on myosin phosphatase target protein (MYPT)-1 phosphorylation, endothelial nitric oxide synthase (eNOS) phosphorylation, and vascular endothelial growth factor (VEGF) levels in retinas of diabetic mice, on NF-κβ activity and ICAM-1 expression in endothelial cells (ECs). In vivo, AMA0428 significantly reduced vessel leakage and neovascularization, respectively, in the STZ and OIR model, comparable to DC101 therapy. Additionally, the ROCK inhibitor decreased neurodegeneration in both models and inhibited leukostasis by 30% (p < 0.05) in the STZ model (p < 0.05), while DC101 had no positive effect on the outcome of these latter processes. ROCK activity was upregulated in the diabetic retina and AMA0428 administration resulted in

  9. Geochronology and geochemistry of early Paleozoic intrusive rocks from the Khanka Massif in the Russian Far East: Petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Xu, Ting; Xu, Wen-Liang; Wang, Feng; Ge, Wen-Chun; Sorokin, A. A.

    2018-02-01

    This paper presents new geochronological and geochemical data for early Paleozoic intrusive rocks from the Khanka Massif in the Russian Far East, with the aim of elucidating the Paleozoic evolution and tectonic attributes of the Khanka Massif. New U-Pb zircon data indicate that early Paleozoic magmatism within the Khanka Massif can be subdivided into at least four stages: 502, 492, 462-445, and 430 Ma. The 502 Ma pyroxene diorites contain 58.28-59.64 wt% SiO2, 2.84-3.69 wt% MgO, and relatively high Cr and Ni contents. Negative εHf(t) values (- 1.8 to - 0.4), along with other geochemical data, indicate that the primary magma was derived from partial melting of mafic lower crust with the addition of mantle material. The 492 Ma syenogranites have high SiO2 and K2O contents, and show positive Eu anomalies, indicating the primary magma was generated by partial melting of lower crust at relatively low pressure. The 445 Ma Na-rich trondhjemites display high Sr/Y ratios and positive εHf(t) values (+ 1.8 to + 3.9), indicating the primary magma was generated by partial melting of thickened hydrous mafic crust. The 430 Ma granitoids have high SiO2 and K2O contents, zircon εHf(t) values of - 5.4 to + 5.8, and two-stage model ages of 1757-1045 Ma, suggesting the primary magma was produced by partial melting of heterogeneous Proterozoic lower crustal material. The geochemistry of these early Paleozoic intrusive assemblages indicates their formation in an active continental margin setting associated with the subduction of a paleo-oceanic plate beneath the Khanka Massif. The εHf(t) values show an increasingly negative trend with increasing latitude, revealing a lateral heterogeneity of the lower crust beneath the Khanka Massif. Regional comparisons of the magmatic events indicate that the Khanka Massif in the Russian Far East has a tectonic affinity to the Songnen-Zhangguangcai Range Massif rather than the adjacent Jiamusi Massif.

  10. Proterozoic orogens in southern Peninsular India: Contiguities and complexities

    NASA Astrophysics Data System (ADS)

    Chetty, T. R. K.; Santosh, M.

    2013-12-01

    The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents - Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ˜400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P-T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale "flower structures", transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a

  11. Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

    Aleinikoff, J.N.; Horton, J. Wright; Walter, M.

    1996-01-01

    Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12-29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38-52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ?? 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ?? 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ?? 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

  12. Peroxy defects in Rocks and H2O2 formation on the early Earth

    NASA Astrophysics Data System (ADS)

    Gray, A.; Balk, M.; Mason, P.; Freund, F.; Rothschild, L.

    2013-12-01

    An oxygen-rich atmosphere appears to have been a prerequisite for complex life to evolve on Earth and possibly elsewhere in the Universe. The question is still shrouded in uncertainty how free oxygen became available on the early Earth. Here we study processes of peroxy defects in silicate minerals which, upon weathering, generate mobilized electronic charge carriers resulting in oxygen formation in an initially anoxic subsurface environment. Reactive Oxygen Species (ROS) are precursors to molecular oxygen during this process. Due to their toxicity they may have strongly influenced the evolution of life. ROS are generated during hydrolysis of peroxy defects, which consist of pairs of oxygen anions. A second pathway for formation occurs during (bio) transformations of iron sulphide minerals. ROS are produced and consumed by intracellular and extracellular reactions of Fe, Mn, C, N, and S species. We propose that despite an overall reducing or neutral oxidation state of the macroenvironment and the absence of free O2 in the atmosphere, microorganisms on the early Earth had to cope with ROS in their microenvironments. They were thus under evolutionary pressure to develop enzymatic and other defenses against the potentially dangerous, even lethal effects of ROS and oxygen. We have investigated how oxygen might be released through weathering and test microorganisms in contact with rock surfaces. Our results show how early Life might have adapted to oxygen. Early microorganisms must have "trained" to detoxify ROS prior to the evolution of aerobic metabolism and oxygenic photosynthesis. A possible way out of this dilemma comes from a study of igneous and high-grade metamorphic rocks, whose minerals contain a small but significant fraction of oxygen anions in the valence state 1- , forming peroxy links of the type O3Si-OO-SiO3 [1, 2]. As water hydrolyzes the peroxy links hydrogen peroxide, H2O2, forms. Continued experimental discovery of H2O2 formation at rock

  13. Precious metals associated with Late Cretaceous-early Tertiary igneous rocks of southwestern Alaska

    Bundtzen, Thomas K.; Miller, Marti L.; Goldfarb, Richard J.; Miller, Lance D.

    1997-01-01

    Placer gold and precious metal-bearing lode deposits of southwestern Alaska lie within a region 550 by 350 km, herein referred to as the Kuskokwim mineral belt. This mineral belt has yielded 100,240 kg (3.22 Moz) of gold, 12, 813 kg (412,000 oz) of silver, 1,377,412 kg (39,960 flasks) of mercury, and modest amounts of antimony and tungsten derived primarily from the late Cretaceous-early Tertiary igneous complexes of four major types: (1) alkali-calcic, comagmatic volcanic-plutonic complexes and isolated plutons, (2) calc-alkaline, meta-aluminous reduced plutons, (3) peraluminous alaskite or granite-porphyry sills and dike swarms, and (4) andesite-rhyolite subaerial volcanic rocks.About 80 percent of the 77 to 52 Ma intrusive and volcanic rocks intrude or overlie the middle to Upper Cretaceous Kuskokwim Group sedimentary and volcanic rocks, as well as the Paleozoic-Mesozoic rocks of the Nixon Fork, Innoko, Goodnews, and Ruby preaccretionary terranes.The major precious metal-bearing deposit types related to Late Cretaceous-early Tertiary igneous complexes of the Kuskokwim mineral belt are subdivided as follows: (1) plutonic-hosted copper-gold polymetallic stockwork, skarn, and vein deposits, (2) peraluminous granite-porphory-hosted gold polymetallic deposits, (3) plutonic-related, boron-enriched silver-tin polymetallic breccia pipes and replacement deposits, (4) gold and silver mineralization in epithermal systems, and (5) gold polymetallic heavy mineral placer deposits. Ten deposits genetically related to Late Cretaceous-early Tertiary intrusions contain minimum, inferred reserves amounting to 162,572 kg (5.23 Moz) of gold, 201,015 kg (6.46 Moz) silver, 12,160 metric tons (t) of tin, and 28,088 t of copper.The lodes occur in veins, stockworks, breccia pipes, and replacement deposits that formed in epithermal to mesothermal temperature-pressure conditions. Fluid inclusion, isotopic age, mineral assemblage, alteration assemblage, and structural data indicate that

  14. A Laughing Gas Greenhouse for the Proterozoic?

    NASA Astrophysics Data System (ADS)

    Roberson, A. L.; Roadt, J.; Halevy, I.; Kasting, J. F.

    2010-12-01

    An anoxic, sulfidic ‘Canfield ocean’ during the Proterozoic (0.75-2.3 Ga) would have had limited trace metal abundances because of the low solubility of metal sulfides. The limitation on copper, specifically, would have had a significant impact on marine denitrification. Copper is needed for the enzyme that controls the final step of denitrification, from N2O to N2. Today, only about 5-6 percent of denitrification results in release of N2O. If all denitrification stopped at N2O during the Proterozoic, the N2O flux could have been 15-20 times higher than today. Other parts of the nitrogen cycle should have been able to operate at rates comparable to today, as catalysts for these reactions should have existed. The high N2O flux should have created higher atmospheric N2O concentrations; although this effect may have been offset by faster rates of N2O photolysis if O2 concentrations were lower than today. N2O concentrations of 0.3 to 30 ppmv, along with methane levels of 30-100 ppm, could have kept the surface warm during the Proterozoic without necessitating high CO2 levels. The high methane concentrations were a consequence of lack of dissolved O2 and sulfate in the deep ocean, which should have led to a high CH4 flux from marine sediments. A second oxygenation event at the end of the Proterozoic would have resulted in a shift to a more modern ocean and, consequently, more modern concentrations of atmospheric N2O and CH4.

  15. Testing the Mojave-Sonora megashear hypothesis: Evidence from Paleoproterozoic igneous rocks and deformed Mesozoic strata in Sonora, Mexico

    Amato, J.M.; Lawton, T.F.; Mauel, D.J.; Leggett, W.J.; Gonzalez-Leon, C. M.; Farmer, G.L.; Wooden, J.L.

    2009-01-01

    U-Pb ages and Nd isotope values of Proterozoic rocks in Sonora, Mexico, indicate the presence of Caborca-type basement, predicted to lie only south of the Mojave-Sonora mega-shear, 40 km north of the postulated megashear. Granitoids have U-Pb zircon ages of 1763-1737 Ma and 1076 Ma, with ??Nd(t) values from +1.4 to -4.3, typical of the Caborca block. Lower Jurassic strata near the Proterozoic rocks contain large granitic clasts with U-Pb ages and ??Nd(t) values indistinguishable from those of Caborcan basement. Caborca-type basement was thus present at this location north of the megashear by 190 Ma, the depositional age of the Jurassic strata. The Proterozoic rocks are interpreted as parautochthonous, exhumed and juxtaposed against the Mesozoic section by a reverse fault that formed a footwall shortcut across a Jurassic normal fault. Geochronology, isotope geochemistry, and structural geology are therefore inconsistent with Late Jurassic megashear displacement and require either that no major transcurrent structure is present in Sonora or that strike-slip displacement occurred prior to Early Jurassic time. ?? 2009 The Geological Society of America.

  16. Saddle-shaped reticulate Nummulites from Early Oligocene rocks of Khari area, SW Kutch, India

    NASA Astrophysics Data System (ADS)

    Sengupta, S.; Sarkar, Sampa; Mukhopadhyay, S.

    2011-04-01

    Saddle-shaped reticulate Nummulites from the Early Oligocene rocks of Khari area, SW Kutch, India is reported here for the first time. Unusual shape of this Nummulites is due to the curved nature of the coiling plane, indicating space constrained postembryonic test growth. With regular development of chambers, septa and septal filaments, the saddle-shaped Nummulites constitutes the third morphotype of N. cf. fichteli Michelotti form A. Other morphotypes of the species reported earlier include inflated lenticular and conical tests. Multiple morphotypes of N. cf. fichteli form A indicates varied test growth in response to substrate conditions. Morphological variability exhibited by N. cf. fichteli form A from Kutch and some Early Oligocene reticulate Nummulites from the Far East are comparable. This faunal suite is morphologically distinct from the contemporary reticulate Nummulites of the European localities.

  17. Paleomagnetism of Proterozoic mafic dikes from the Tobacco Root Mountains, southwest Montana

    Harlan, S.S.; Geissman, J. Wm; Snee, L.W.

    2008-01-01

    Mountains dikes provide evidence that they were partially to completely remagnetized during latest Cretaceous to early Tertiary time, perhaps due to thermal affects associated with emplacement of the Late Cretaceous Tobacco Root Batholith. The overall agreement of paleomagnetic poles from the Proterozoic dikes with those of age equivalent rocks elsewhere in North America and agreement of the secondary magnetization with expected directions for the latest Cretaceous/early Tertiary indicate that the rocks of the Tobacco Root Mountains have not experienced significant tilting or vertical axis rotation since the Mesoproterozoic. The new paleomagnetic poles from this study thus provide key data for refining Meso- and Neoproterozoic parts of the North American APW path. ?? 2008 Elsevier B.V. All rights reserved.

  18. Water-Rock Reactions on Non-Planetary Bodies in the Early Solar System

    NASA Astrophysics Data System (ADS)

    Zolotov, M. Y.

    2005-12-01

    Reactions of aqueous fluids with rocks shortly after formation of the solar system affected the oxidation states, mineralogy, organic speciation, ice composition, and surface/atmospheric chemistry of asteroids, icy satellites of giant plants, and possibly Kuiper belt objects. Water condensed as ice in the solar nebula, was incorporated into the composition of these bodies together with rocky components represented by extremely reduced and anhydrous nebular condensates (e.g., Fe-rich metal, forsterite, low-Ca pyroxene, troilite, Ca-Mg-Al oxides, phosphides), presolar grains (SiC, graphite, diamond, Al-, Mg-, Ti-oxides) and organic compounds and polymers. Radioactive decay of short-lived radionuclides on small bodies, and accretionary heat and decay of long-lived radionuclides on large bodies provided energy to melt ice. On smaller bodies, low gravity precluded separation of water from rocks and restricted fluid dynamics. On larger bodies, water was separated from descending rocks, limiting the duration of water-rock reactions. Competitive oxidation and hydration by water affected both inorganic and organic compounds in rocks. Oxidation of minerals led to formation of ferrous silicates, magnetite, pyrrhotite, Ni sulfides, Ni-rich metal alloys, chromite, phosphates, carbonates and sulfates. Hydration caused formation of phyllosilicates (serpentine, chlorites, smectite clays, amphiboles, and micas), hydroxides, and hydrated sulfides and salts. High water/rock ratios, elevated temperatures and low pressures favored oxidation. Low temperatures supported hydration. In some icy satellites (Europa, Ganymede) high water content and hydrothermal processes during differentiation may have caused profound oxidation leading to carbonates and even sulfates. Since water was the only early oxidizing agent, the elevated oxidation state of Io implies its early aqueous history. Hydrogen was produced in all oxidation reactions and preferentially separated into the gas phase. Escape of H

  19. Proterozoic Milankovitch cycles and the history of the solar system.

    PubMed

    Meyers, Stephen R; Malinverno, Alberto

    2018-06-19

    The geologic record of Milankovitch climate cycles provides a rich conceptual and temporal framework for evaluating Earth system evolution, bestowing a sharp lens through which to view our planet's history. However, the utility of these cycles for constraining the early Earth system is hindered by seemingly insurmountable uncertainties in our knowledge of solar system behavior (including Earth-Moon history), and poor temporal control for validation of cycle periods (e.g., from radioisotopic dates). Here we address these problems using a Bayesian inversion approach to quantitatively link astronomical theory with geologic observation, allowing a reconstruction of Proterozoic astronomical cycles, fundamental frequencies of the solar system, the precession constant, and the underlying geologic timescale, directly from stratigraphic data. Application of the approach to 1.4-billion-year-old rhythmites indicates a precession constant of 85.79 ± 2.72 arcsec/year (2σ), an Earth-Moon distance of 340,900 ± 2,600 km (2σ), and length of day of 18.68 ± 0.25 hours (2σ), with dominant climatic precession cycles of ∼14 ky and eccentricity cycles of ∼131 ky. The results confirm reduced tidal dissipation in the Proterozoic. A complementary analysis of Eocene rhythmites (∼55 Ma) illustrates how the approach offers a means to map out ancient solar system behavior and Earth-Moon history using the geologic archive. The method also provides robust quantitative uncertainties on the eccentricity and climatic precession periods, and derived astronomical timescales. As a consequence, the temporal resolution of ancient Earth system processes is enhanced, and our knowledge of early solar system dynamics is greatly improved.

  20. Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

    Barth, Andrew P.; Wooden, Joseph L.; Coleman, Drew S.; Fanning, C. Mark

    2000-01-01

    The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

  1. Hydrocarbons related to early Cretaceous source rocks, reservoirs and seals, trapped in northeastern Neuqun basin, Argentina

    SciT

    Gulisano, C.; Minniti, S.; Rossi, G.

    1996-08-01

    The Jurassic-Cretaceous backarc Neuqun Basin, located in the west central part of Argentina, is currently the most prolific oil basin of the country. The primary objective of this study is to evaluate an Early Cretaceous to Tertiary petroleum system in the northeastern portion of the basin, where oil and gas occurrences (e.g., Puesto Hernandez, Chihuido de la Sierra Negra, El Trapial and Filo Morado oil fields, among others) provide 82 MMBO/yr comprising 67% of the basin oil production and 31% of Argentina. The source rocks are represented by two thick sections of basinal kerogen type I and II organic-rich shales,more » deposited during transgressive peaks (Agrio Formation), with TOC content up to 5.1%. Lowstand sandstones bodies, 10 to 100 m thick, are composed of eolian and fluvial facies with good reservoir conditions (Avil and Troncoso Sandstones). The seals are provided by the organic-rich shales resting sharply upon the Avil Sandstone and a widespread Aptian-Albian evaporitic event (Huitrin Formation) on top of the Troncoso reservoir. Tertiary structural traps (duplex anticlines) are developed in the outer foothills, whereas structural, combined and stratigraphic traps are present in the adjacent stable structural platform. Oil-to-source rock and oil-to-oil correlation by chromatographic and biomarker fingerprints, carbon isotopic composition and the geological evidences support the proposed oil system.« less

  2. Early Cretaceous bimodal volcanic rocks in the southern Lhasa terrane, south Tibet: Age, petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Wang, Chao; Ding, Lin; Liu, Zhi-Chao; Zhang, Li-Yun; Yue, Ya-Hui

    2017-01-01

    Limited geochronological and geochemical data from Early Cretaceous igneous rocks of the Gangdese Belt have resulted in a dispute regarding the subduction history of Neo-Tethyan Ocean. To approach this issue, we performed detailed in-situ zircon U-Pb and Hf isotopic, whole-rock elemental and Sr-Nd isotopic analyses on Late Mesozoic volcanic rocks exposed in the Liqiongda area, southern Lhasa terrane. These volcanic rocks are calc-alkaline series, dominated by basalts, basaltic andesites, and subordinate rhyolites, with a bimodal suite. The LA-ICPMS zircon U-Pb dating results of the basaltic andesites and rhyolites indicate that these volcanic rocks erupted during the Early Cretaceous (137-130 Ma). The basaltic rocks are high-alumina (average > 17 wt.%), enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depleted in high field strength elements (HFSEs), showing subduction-related characteristics. They display highly positive zircon εHf(t) values (+ 10.0 to + 16.3) and whole-rock εNd(t) values (+ 5.38 to + 7.47). The silicic suite is characterized by low Al2O3 (< 15.4 wt.%), Mg# (< 40), and TiO2 (< 0.3 wt.%) abundances; enriched and variable concentrations of LILEs and REEs; and strongly negative Eu anomalies (Eu/Eu* = 0.08-0.19), as well as depleted Hf isotopic compositions (εHf(t) = + 4.9 to + 16.4) and Nd isotopic compositions (εNd(t) = + 5.26 to + 6.71). Consequently, we envision a process of basaltic magmas similar to that of MORB extracted from a source metasomatized by slab-derived components for the petrogenesis of mafic rocks, whereas the subsequent mafic magma underplating triggered partial melting of the juvenile crust to generate acidic magma. Our results confirm the presence of Early Cretaceous volcanism in the southern Lhasa terrane. Combined with the distribution of the contemporary magmatism, deformation style, and sedimentary characteristics in the Lhasa terrane, we favor the suggestion that the Neo

  3. Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate

    NASA Astrophysics Data System (ADS)

    Hardisty, Dalton S.; Lu, Zunli; Bekker, Andrey; Diamond, Charles W.; Gill, Benjamin C.; Jiang, Ganqing; Kah, Linda C.; Knoll, Andrew H.; Loyd, Sean J.; Osburn, Magdalena R.; Planavsky, Noah J.; Wang, Chunjiang; Zhou, Xiaoli; Lyons, Timothy W.

    2017-04-01

    The Proterozoic Eon hosted the emergence and initial recorded diversification of eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than today's, although how much lower is debated. Here, we use concentrations of iodate (the oxidized iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both local oxygen availability and the relative proximity to anoxic waters. To assess the validity of our approach, Neogene-Quaternary carbonates are used to demonstrate that diagenesis most often decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo- and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene-Quaternary carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow oxic-anoxic interface. Oxygen concentrations in surface waters would have at least intermittently been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time intervals, are consistent with a dynamic chemocline and anoxic waters that would have episodically mixed upward and laterally into the shallow oceans. This redox instability may have challenged early eukaryotic diversification and expansion, creating an evolutionary landscape unfavorable for the emergence of animals.

  4. Proterozoic microfossils revealing the time of algal divergences

    NASA Astrophysics Data System (ADS)

    Moczydlowska-Vidal, Malgorzata

    2010-05-01

    Proterozoic microfossils revealing the time of algal divergences Małgorzata Moczydłowska-Vidal Uppsala University, Department of Earth Sciences, Palaeobiology, Villavägen 16, SE 752 36 Uppsala, Sweden (malgo.vidal@pal.uu.se) Morphological and reproductive features and cell wall ultrastructure and biochemistry of Proterozoic acritarchs are used to determine their affinity to modern algae. The first appearance datum of these microbiota is traced to infer a minimum age of the divergence of the algal classes to which they may belong. The chronological appearance of microfossils that represent phycoma-like and zygotic cysts and vegetative cells and/or aplanospores, respectively interpreted as prasinophyceaen and chlorophyceaen microalgae, is related to the Viridiplantae phylogeny. These divergence times differ from molecular clock estimates, and the palaeontological evidence suggests that they are older. The best examples of unicellular, organic-walled microfossils (acritarchs) from the Mesoproterozoic to Early Ordovician are reviewed to demonstrate features, which are indicative of their affinity to photosynthetic microalgae. The first indication that a microfossil may be algal is a decay- and acid-resistant cell wall, which reflects its biochemistry and ultrastructure, and probably indicates the ability to protect a resting/reproductive cyst. The biopolymers synthesized in the cell walls of algae and in land plants ("plant cells"), such as sporopollenin/algaenan, are diagnostic for photosynthetic taxa and were inherited from early unicellular ancestors. These preservable cell walls are resistant to acetolysis, hydrolysis and acids, and show diagnostic ultrastructures such as the trilaminar sheath structure (TLS). "Plant cell" walls differ in terms of chemical compounds, which give high preservation potential, from fungal and animal cell walls. Fungal and animal cells are fossilized only by syngenetic permineralization, whereas "plant cells" are fossilized as body

  5. Naturally occurring contaminants in the Piedmont and Blue Ridge crystalline-rock aquifers and Piedmont Early Mesozoic basin siliciclastic-rock aquifers, eastern United States, 1994–2008

    Chapman, Melinda J.; Cravotta, Charles A.; Szabo, Zoltan; Lindsay, Bruce D.

    2013-01-01

    Groundwater quality and aquifer lithologies in the Piedmont and Blue Ridge Physiographic Provinces in the eastern United States vary widely as a result of complex geologic history. Bedrock composition (mineralogy) and geochemical conditions in the aquifer directly affect the occurrence (presence in rock and groundwater) and distribution (concentration and mobility) of potential naturally occurring contaminants, such as arsenic and radionuclides, in drinking water. To evaluate potential relations between aquifer lithology and the spatial distribution of naturally occurring contaminants, the crystalline-rock aquifers of the Piedmont and Blue Ridge Physiographic Provinces and the siliciclastic-rock aquifers of the Early Mesozoic basin of the Piedmont Physiographic Province were divided into 14 lithologic groups, each having from 1 to 16 lithochemical subgroups, based on primary rock type, mineralogy, and weathering potential. Groundwater-quality data collected by the U.S. Geological Survey (USGS) National Water-Quality Assessment (NAWQA) Program from 1994 through 2008 from 346 wells and springs in various hydrogeologic and land-use settings from Georgia through New Jersey were compiled and analyzed for this study. Analyses for most constituents were for filtered samples, and, thus, the compiled data consist largely of dissolved concentrations. Concentrations were compared to criteria for protection of human health, such as U.S. Environmental Protection Agency (USEPA) drinking water maximum contaminant levels and secondary maximum contaminant levels or health-based screening levels developed by the USGS NAWQA Program in cooperation with the USEPA, the New Jersey Department of Environmental Protection, and Oregon Health & Science University. Correlations among constituent concentrations, pH, and oxidation-reduction (redox) conditions were used to infer geochemical controls on constituent mobility within the aquifers. Of the 23 trace-element constituents evaluated

  6. Late Proterozoic charnockites in Orissa, India: A U-Pb and Rb-Sr isotopic study

    SciT

    Aftalion, M.; Bowes, D.R.; Dash, B.

    1988-11-01

    Charnockite formation in the Angul district of Orissa took place between 1088 + 26/ -17 Ma, the U-Pb zircon upper intercept crystallization age of a leptynite neosome, and 957 +8/ -4-956 {plus minus} 4 Ma, the U-Pb zircon-monazite upper intercept and U-Pb monazite crystallization ages of a granite. Confirmation of the Proterozoic age of the charnockites is given by (1) a U-Pb zircon upper intercept 1159 + 59/ -30 Ma age and a Rb-Sr whole-rock 1080 {plus minus} 65 Ma age for an augen gneiss which pre-dates the leptynite, and (2) U-Pb monazite ages of 973 {plus minus} 5,964 {plusmore » minus} 4, and 953 {plus minus} 4 Ma for a gray quartzofeldspathic gneiss, the augen gneiss, and the leptynite, respectively: these late Proterozoic dates are interpreted as representing ages recorded during charnockitization. The ca. 950-980 Ma charnockite- and granite-forming events are related to the evolution of mantle-derived, CO{sub 2}-bearing basic magma emplaced into the deeper levels of an extensional tectonic-transcurrent fault regime. The ca. 1100-1150 Ma tectonothermal and igneous events represent compressional tectonism in reactivated crystalline basement in the late mid-Proterozoic Eastern Ghats orogenic belt.« less

  7. Microfossils from oolites and pisolites of the Upper Proterozoic Eleonore Bay Group, Central East Greenland

    NASA Technical Reports Server (NTRS)

    Green, J. W.; Knoll, A. H.; Swett, K.

    1988-01-01

    Silicified oolites and pisolites from Bed 18 of the Upper Proterozoic (about 700-800 Ma) Limestone-Dolomite "Series" of the Eleonore Bay Group, central East Greenland, contain a diverse suite of organically preserved microfossils that is, for the most part. [Of the] assemblages previously described from Proterozoic cherts and shales. Three principal assemblages occur in these rocks: 1) a class bound assemblage found in detrital carbonate grains (now silicified) that served as nuclei for ooid and pisoid growth, as well as in uncoated mud and mat clasts that were carried into the zone of ooid and pisoid deposition; 2) an epilithic and interstitial assemblage consisting of microorganisms that occurred on top of and between grains; and 3) a euendolithic assemblage composed of microbes that actively bored into coated grains. The Upper Proterozoic euendolithic assemblage closely resembles a community of euendolithic cyanobacteria found today in shallow marine ooid sands of the Bahama Banks. Thirteen species are described, of which eight are new, five representing new genera: Eohyella dichotoma n. sp., Eohyella endoatracta n. sp., Eohyella rectoclada n. sp., Thylacocausticus globorum n. gen. and sp., Cunicularius halleri n. gen. and sp., Graviglomus incrustus n. gen. and sp., Perulagranum obovatum n. gen. and sp., and Parenchymodiscus endolithicus n. gen. and sp.

  8. Petrogenesis of siliceous high-Mg series rocks as exemplified by the Early Paleoproterozoic mafic volcanic rocks of the Eastern Baltic Shield: enriched mantle versus crustal contamination

    NASA Astrophysics Data System (ADS)

    Bogina, Maria; Zlobin, Valeriy; Sharkov, Evgenii; Chistyakov, Alexeii

    2015-04-01

    The Early Paleoproterozoic stage in the Earth's evolution was marked by the initiation of global rift systems, the tectonic nature of which was determined by plume geodynamics. These processes caused the voluminous emplacement of mantle melts with the formation of dike swarms, mafic-ultramafic layered intrusions, and volcanic rocks. All these rocks are usually considered as derivatives of SHMS (siliceous high-magnesian series). Within the Eastern Baltic Shield, the SHMS volcanic rocks are localized in the domains with different crustal history: in the Vodlozero block of the Karelian craton with the oldest (Middle Archean) crust, in the Central Block of the same craton with the Neoarchean crust, and in the Kola Craton with a heterogeneous crust. At the same time, these rocks are characterized by sufficiently close geochemical characteristics: high REE fractionation ((La/Yb)N = 4.9-11.7, (La/Sm)N=2.3-3.6, (Gd/Yb)N =1.66-2.74)), LILE enrichment, negative Nb anomaly, low to moderate Ti content, and sufficiently narrow variations in Nd isotope composition from -2.0 to -0.4 epsilon units. The tectonomagmatic interpretation of these rocks was ambiguous, because such characteristics may be produced by both crustal contamination of depleted mantle melts, and by generation from a mantle source metasomatized during previous subduction event. Similar REE patterns and overlapping Nd isotope compositions indicate that the studied basaltic rocks were formed from similar sources. If crustal contamination en route to the surface would play a significant role in the formation of the studied basalts, then almost equal amounts of contaminant of similar composition are required to produce the mafic rocks with similar geochemical signatures and close Nd isotopic compositions, which is hardly possible for the rocks spaced far apart in a heterogeneous crust. This conclusion is consistent with analysis of some relations between incompatible elements and their ratios. In particular, the

  9. Evidence for post-1620 Ma Proterozoic regional deformation, Lucy Gray Range, southern Nevada

    SciT

    Duebendorfer, E.M.; Christensen, C.H.; Shafiqullah, M.

    1993-04-01

    Major mylonite zones in the northern Lucy Gray Range, Nevada, deform and are spatially associated with the 1,425 Ma Beer Bottle Pass pluton, Mylonitic granite yielded a K-Ar biotite date of 1,400 [+-] 30 Ma and is overlain nonconformably by the Cambrian Tapeats Sandstone, thus constraining deformation to the Proterozoic. The mylonites may therefore represent an unrecognized period of Proterozoic deformation in the Southwest. Field and microstructural studies were undertaken to evaluate between 3 possible models for the apparent spatial association of granite and mylonites: (1) deformation directly related to pluton emplacement (ballooning); (2) synkinematic pluton emplacement; or (3) post-emplacementmore » deformation. Mylonite zones up to 50 meters thick strike north to northeast, dip moderately to steeply northwest, and contain a remarkably consistent west-plunging mineral lineation. Mylonites are present locally at the granite-wall rock contact; however, less than 30% of the exposed contact is mylonitic. The authors reject a pluton-emplacement origin for the mylonites because (1) mylonite zones within wall rocks locally strike at high angles to an undeformed pluton-wall rock contact, (2) the consistent (pluton-side-down) shear sense is more compatible with a uniform-sense simple shear zone than a ballooning pluton, (3) plane strain fabrics dominate over flattening fabrics, and (4) mylonites adjacent to pluton contacts lack annealing textures predicted by the ballooning model. If so, the conventional interpretation of 1,400 Ga granitoids as anorogenic may need to be re-evaluated. The authors cannot, however, rule out the possibility that the mylonites completely postdate intrusion of the Beer Bottle Pass pluton. Future work is planned to delimit the regional extent of this previously unrecognized Proterozoic deformational event.« less

  10. Terminal Proterozoic reorganization of biogeochemical cycles

    NASA Technical Reports Server (NTRS)

    Logan, G. A.; Hayes, J. M.; Hieshima, G. B.; Summons, R. E.

    1995-01-01

    The Proterozoic aeon (2,500-540 million years ago) saw episodic increases in atmospheric oxygen content, the evolution of multicellular life and, at its close, an enormous radiation of animal diversity. These profound biological and environmental changes must have been linked, but the underlying mechanisms have been obscure. Here we show that hydrocarbons extracted from Proterozoic sediments in several locations worldwide are derived mainly from bacteria or other heterotrophs rather than from photosynthetic organisms. Biodegradation of algal products in sedimenting matter was therefore unusually complete, indicating that organic material was extensively reworked as it sank slowly through the water column. We propose that a significant proportion of this reworking will have been mediated by sulphate-reducing bacteria, forming sulphide. The production of sulphide and consumption of oxygen near the ocean surface will have inhibited transport of O2 to the deep ocean. We find that preservation of algal-lipid skeletons improves at the beginning of the Cambrian, reflecting the increase in transport by rapidly sinking faecal pellets. We suggest that this rapid removal of organic matter will have increased oxygenation of surface waters, leading to a descent of the O2-sulphide interface to the sea floor and to marked changes in the marine environment, ultimately contributing to the Cambrian radiation.

  11. Palaeomagnetism and geochemistry of Early Palaeozoic rocks of the Barrandian (Teplé-Barrandian Unit, Bohemian Massif): palaeotectonic implications

    NASA Astrophysics Data System (ADS)

    Patočka, F.; Pruner, P.; Štorch, P.

    The Barrandian area (the Teplá-Barrandian unit, Bohemian Massif) provided palaeomagnetic results on Early Palaeozoic rocks and chemical data on siliciclastic sediments of both Middle Cambrian and Early Ordovician to Middle Devonian sedimentary sequences; an outcoming interpretation defined source areas of clastic material and palaeotectonic settings of the siliciclastic rock deposition. The siliciclastic rocks of the earliest Palaeozoic sedimentation cycle, deposited in the Cambrian Příbram-Jince Basin of the Barrandian, were derived from an early Cadomian volcanic island arc developed on Neoproterozoic oceanic lithosphere and accreted to a Cadomian active margin of northwestern Gondwana. Inversion of relief terminated the Cambrian sedimentation, and a successory Prague Basin subsided nearby since Tremadocian. Source area of the Ordovician and Early Silurian shallow-marine siliciclastic sediments corresponded to progressively dissected crust of continental arc/active continental margin type of Cadomian age. Since Late Ordovician onwards both synsedimentary within-plate basic volcanics and older sediments had been contributing in recognizable proportions to the siliciclastic rocks. The siliciclastic sedimentation was replaced by deposition of carbonate rocks throughout late Early Silurian to Early Devonian period of withdrawal of the Cadomian clastic material source. Above the carbonates an early Givetian flysch-like siliciclastic suite completed sedimentation in the Barrandian. In times between Middle Cambrian and Early/Middle Devonian boundary interval an extensional tectonic setting prevailed in the Teplá-Barrandian unit. The extensional regime was related to Early Palaeozoic large-scale fragmentation of the Cadomian belt of northwestern Gondwana and origin of Armorican microcontinent assemblage. The Teplá-Barrandian unit was also engaged in a peri-equatorially oriented drift of Armorican microcontinent assemblage throughout the Early Palaeozoic: respective

  12. Magnetotelluric survey to locate the Archean/Proterozoic suture zone north of Wells, Nevada

    Williams, Jackie M.; Rodriguez, Brian D.

    2006-01-01

    It is important to know whether major mining districts in the Northern Nevada Gold Province are underlain by rocks of the Archean Wyoming craton, which are known to contain orogenic gold deposits, or by accreted rocks of the Paleoproterozoic Mojave province. It is also important to know the location and orientation of the Archean/Proterozoic suture zone between these provinces as well as major basement structures within these terranes because they may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The Archean was the main gold-mineralization period, and Archean lode-gold deposits were formed at mid-crustal depths along major shear zones. The nature of the crystalline basement below the Northern Nevada Gold Province and the location of major faults within it are relevant to Rodinian reconstructions, crustal development, and ore deposit models (e.g., Hofstra and Cline, 2000; Grauch and others, 2003). According to Whitmeyer and Karlstrom (2004), the Archean cratons of the northwestern United States and Canada had stabilized as continental lithosphere by 2.5 Ga, and were rifted and assembled into a large continental mass by 1.8 Ga, to which the 1.73-1.68 Ga Mohave province was accreted by 1.65 Ga. The Archean/Proterozoic suture zone has a west-southwest strike where it is exposed (Reed, 1993) at the eastern Utah and southwestern Wyoming border (Cheyenne Belt) where it is characterized by an up to 7-km-thick mylonite zone (Smithson and Boyd, 1998). In the Great Basin, the strike of the Archean/Proterozoic suture zone is poorly constrained because it is largely concealed below a Neoproterozoic-Paleozoic miogeocline and basin fill. East-west and southwest-northeast strikes for the Archean/Proterozoic suture zone have been inferred based on Sr, Nd, and Pb isotopic compositions of granitoid intrusions (Tosdal and others, 2000). To better constrain the location and strike of the Archean/Proterozoic suture zone below cover

  13. Transfer of Metasupracrustal Rocks to Midcrustal Depths in the North Cascades Continental Magmatic Arc, Skagit Gneiss Complex, Washington

    NASA Astrophysics Data System (ADS)

    Sauer, K. B.; Gordon, S. M.; Miller, R. B.; Vervoort, J. D.; Fisher, C. M.

    2017-12-01

    The metasupracrustal units within the north central Chelan block of the North Cascades Range, Washington, are investigated to determine mechanisms and timescales of supracrustal rock incorporation into the deep crust of continental magmatic arcs. Zircon U-Pb and Hf-isotope analyses were used to characterize the protoliths of metasedimentary and metaigneous rocks from the Skagit Gneiss Complex, metasupracrustal rocks from the Cascade River Schist, and metavolcanic rocks from the Napeequa Schist. Skagit Gneiss Complex metasedimentary rocks have (1) a wide range of zircon U-Pb dates from Proterozoic to latest Cretaceous and (2) a more limited range of dates, from Late Triassic to latest Cretaceous, and a lack of Proterozoic dates. Two samples from the Cascade River Schist are characterized by Late Cretaceous protoliths. Amphibolites from the Napeequa Schist have Late Triassic protoliths. Similarities between the Skagit Gneiss metasediments and accretionary wedge and forearc sediments in northwestern Washington and Southern California indicate that the protolith for these units was likely deposited in a forearc basin and/or accretionary wedge in the Early to Late Cretaceous (circa 134-79 Ma). Sediment was likely underthrust into the active arc by circa 74-65 Ma, as soon as 7 Ma after deposition, and intruded by voluminous magmas. The incorporation of metasupracrustal units aligns with the timing of major arc magmatism in the North Cascades (circa 79-60 Ma) and may indicate a link between the burial of sediments and pluton emplacement.

  14. Microfossils' diversity from the Proterozoic Taoudeni Basin, Mauritania

    NASA Astrophysics Data System (ADS)

    Beghin, Jérémie; Houzay, Jean-Pierre; Blanpied, Christian; Javaux, Emmanuelle

    2014-05-01

    Prokaryotes and microscopic eukaryotes are known to have appeared well before the Cambrian's adaptative radiation which flourished perceptibly as a generalized macroscopic world. What do we know about the trigger events which stimulated eukaryotic diversification during the Proterozoic? Biological innovations or environmental changes, and indeed probably both (Knoll et al., 2006), played a fundamental role controlling this important step of life's evolution on Earth. Javaux (2011), proposed a diversification pattern of early eukaryotes divided into three steps and focusing on different taxonomic levels, from stem group to within crown group, of the domain Eukarya. Here, we present a new, exquisitely preserved and morphologically diverse assemblage of organic-walled microfossils from the 1.1 Ga El Mreiti Group of the Taoudeni Basin (Mauritania). The assemblage includes beautifully preserved microbial mats comprising pyritized filaments, prokaryotic filamentous sheaths and filaments, microfossils of uncertain biological affinity including smooth isolated and colonial sphaeromorphs (eukaryotes and/or prokaryotes), diverse protists (ornamented and process-bearing acritarchs), as well multicellular microfossils interpreted in the literature as possible xanthophyte algae. Several taxa are reported for the first time in Africa, but are known worldwide. This study improves microfossil diversity previously reported by Amard (1986) and shows purported xanthophyte algae contrary to a previous biomarker study suggesting the absence of eukaryotic algae, other than acritarchs, in the basin (Blumenberg et al., 2012). This new microfossil assemblage and others provide, all together, evidences of early and worldwide diversification of eukaryotes. Thereby, those first qualitative results also provide a basis for further and larger quantitative studies on the Taoudeni Basin. To better understand the palaeobiology (stem or crown group, aerobic or anaerobic metabolism) and

  15. Detrital zircon U-Pb geochronology and whole-rock Nd-isotope constraints on sediment provenance in the Neoproterozoic Sergipano orogen, Brazil: From early passive margins to late foreland basins

    NASA Astrophysics Data System (ADS)

    Oliveira, E. P.; McNaughton, N. J.; Windley, B. F.; Carvalho, M. J.; Nascimento, R. S.

    2015-11-01

    SHRIMP U-Pb detrital zircon geochronology and depleted-mantle Nd-model ages of clastic rocks were combined to understand the sediment provenance in the Neoproterozoic Sergipano Belt. The Sergipano is the main orogenic belt between the Borborema province and the São Francisco Craton, eastern South America; it is divisible into several lithostratigraphic domains from North to South: Canindé, Poço Redondo-Marancó, Macururé, Vaza Barris, and Estância. Nd model ages (TDM) and detrital zircon U-Pb SHRIMP geochronology indicate that the protoliths of clastic metasedimentary rocks from the Marancó and Macururé domains were mostly derived from eroded late Mesoproterozoic to early Neoproterozoic rocks (1000-900 Ma), whereas detritus of similar rocks from the Canindé domain came from a younger source (ca. 700 Ma and 1000 Ma). Samples from the Vaza Barris domain show the greatest scatter of both TDM and zircon ages amongst all domains, but with important contributions from Proterozoic sources (690-1050 Ma and ca. 2100 Ma) and less from Archaean sources. The Estância domain samples have zircon population peaks at 570 Ma, 600 Ma, and 920-980 Ma, with a few older grains; one diamictite contains only ca. 2150 Ma zircon grains. Our preliminary results support a model in which sediments of the Marancó and Macururé domains were deposited on a continental margin of the ancient Borborema plate before its collision with the São Francisco Craton; the Canindé domain is likely to be an aborted Neoproterozoic rift assemblage within the southern part of the Borborema plate (Pernambuco-Alagoas massif). The basal units of the Vaza Barris and Estância domains have clast sources from the São Francisco Craton and are best interpreted as passive margin sediments. However, the uppermost units of the Estância and Vaza Barris domains come from foreland basins formed during collision of Borborema plate with the São Francisco Craton.

  16. Lower Brioverian formations (Upper Proterozoic) of the Armorican Massif (France): geodynamic evolution of source areas revealed by sandstone petrography and geochemistry

    NASA Astrophysics Data System (ADS)

    Dabard, Marie Pierre

    1990-11-01

    Formations with interbedded cherts constitute an important part of the Lower Brioverian succession (Upper Proterozoic age) in the Armorican Massif (northwest France). These formations are composed of shale-sandstone alternations with interbedded siliceous carbonaceous members. Petrographic and geochemical study of the detrital facies shows that these rocks are compositionally immature. The wackes are rich in lithic fragments (volcanic fragments: 3-20% modal; sedimentary and metamorphic fragments: 0-7% modal) and in feldspar (5-16%). From the geochemical point of view, they are relatively enriched in Fe 2+MgO (about 5.5%) and in alkalis with {Na 2O }/{K 2O } ratios greater than 1. The CaO contents are low (about 0.3%). Slightly negative Eu anomalies are observed ( {Eu}/{Eu ∗} = 0.8 ). Their chemical compositions are in agreement with a dominantly acidic source area with deposition in a continental active margin setting. Compared with other Upper Proterozoic deposits of the Armorican Massif, the interbedded-chert formations appear rather similar to other deposits in North Brittany which accumulated in an intra-arc or back-arc basin environment. The formations with interbedded cherts are interpreted as having been deposited during an early stage of magmatic arc activity (around 640-630 Ma ago) in an immature marginal basin. The clastic supply to these formations is derived in part from early volcanic products (acidic to intermediate) which are linked to subduction beneath the North Armorican Domain. Another component is inherited from the reworking of 2000 Ma old basement relics. The opening of the back-arc domain, with associated basaltic volcanism, would bring about a progressive displacement of the interbedded-chert depositional basin towards the continental margin.

  17. U-Pb dating and isotopic signature of the alkaline ring complexes of Bou Naga (Mauritania): its bearing on late proterozoic plate tectonics around the West African craton

    NASA Astrophysics Data System (ADS)

    Blanc, A.; Bernard-Griffiths, J.; Caby, R.; Caruba, C.; Caruba, R.; Dars, R.; Fourcade, S.; Peucat, J. J.

    1992-04-01

    In the West African fold belt of Mauritania, high-grade metamorphic series, similar to those of Amsaga (Reguibat shield-West African Craton), are exposed in a window. At Bou Naga-Mauritania (19° N, 13° 15' W) in the South of this window, an alkaline ring complex has intruded the metamorphic country rocks. This complex consists of two geological formations: the Eastern formation is mainly composed of red rhyolite sills, whereas the Western formation is made up of several kinds of alkaline rocks both saturated and under-saturated which cross cut the earlier saturated units. Three U-Pb zircon age measurements have been made on the alkaline complex, and one on an orthogneiss from the metamorphic country rocks. The syenite and the alkaline granite of the Western block are 676 ± 8 and 687 ± 5 Ma old. The orthogneiss is Archaean with an age of 2709 ± 136 Ma, but the lower intercept of discordia on concordia, shows an age of 756 ± 25 Ma linked with the genesis of the alkaline complex. A major crustal contribution is recorded by Nd and O isotopes in the SiO 2-saturated rocks. These results provide evidence for the correlation of the metamorphic country rocks with the Reguibat Archaean basement and for an early Pan-African continental rifting phase in this area before the tectonometamorphic events in the Mauritanide belt. Furthermore, with regards with previous geodynamic works of the West African Craton, our results leads us to suggest a significant diachronism between late Proterozoic crustal evolution to the West and to the East of the West African Craton. This is a further evidence for modern-type plate tectonics at this time.

  18. Sr, Nd and Pb isotopes in Proterozoic intrusives astride the Grenville Front in Labrador: Implications for crustal contamination and basement mapping

    Ashwal, L.D.; Wooden, J.L.; Emslie, R.F.

    1986-01-01

    We report Sr, Nd and Pb isotopic compositions of mid-Proterozoic anorthosites and related rocks (1.45-1.65 Ga) and of younger olivine diabase dikes (1.4 Ga) from two complexes on either side of the Grenville Front in Labrador. Anorthositic or diabasic samples from the Mealy Mountains (Grenville Province) and Harp Lake (Nain-Churchill Provinces) complexes have very similar major, minor and trace element compositions, but distinctly different isotopic signatures. All Mealy Mountains samples have ISr = 0.7025-0.7033, ??{lunate}Nd = +0.6 to +5.6 and Pb isotopic compositions consistent with derivation from a mantle source depleted with respect to Nd/Sm and Rb/Sr. Pb isotopic compositions for the Mealy Mountains samples are slightly more radiogenic than model mantle compositions. All Harp Lake samples have ISr = 0.7032-0.7066, ??{lunate}Nd = -0.3 to -4.4 and variable, but generally unradiogenic 207Pb 204Pb and 206Pb 204Pb compared to model mantle, suggesting mixing between a mantle-derived component and a U-depleted crustal contaminant. Crustal contaminants are probably a variety of Archean high-grade quartzofeldspathic gneisses with low U/Pb ratios and include a component that must be isotopically similar to the early Archean (>3.6 Ga) Uivak gneisses of Labrador or the Amitsoq gneisses of west Greenland. This would imply that the ancient gneiss complex of coastal Labrador and Greenland is larger than indicated by present surface exposure and may extend in the subsurface as far west as the Labrador Trough. If Harp Lake and Mealy Mountains samples were subjected to the same degree of contamination, as suggested by their chemical similarities, then the Mealy contaminants must be much younger, probably early or middle Proterozoic in age. The Labrador segment of the Grenville Front, therefore, appears to coincide with the southern margin of the Archean North Atlantic craton and may represent a pre mid-Proterozoic suture. ?? 1986.

  19. Tracking the Archean-Proterozoic suture zone in the northeastern Great Basin, Nevada and Utah

    Rodriguez, B.D.; Williams, J.M.

    2008-01-01

    It is important to know whether major mining districts in north-central Nevada are underlain by crust of the Archean Wyoming craton, known to contain major orogenic gold deposits or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between these provinces is also important because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. The suture zone is exposed in northeastern Utah and south-western Wyoming and exhibits a southwest strike. In the Great Basin, the suture zone strike is poorly constrained because it is largely concealed below a Neoproterozoic-Paleozoic miogeocline and Cenozoic basin fill. Two-dimensional resistivity modeling of three regional north-south magnetotelluric sounding profiles in western Utah, north-central Nevada, and northeastern Nevada, and one east-west profile in northeastern Nevada, reveals a deeply penetrating (>10 km depth), broad (tens of kilometers) conductor (1-20 ohm-meters) that may be the Archean-Proterozoic suture zone, which formed during Early Proterozoic rifting of the continent and subsequent Proterozoic accretion. This major crustal conductor changes strike direction from southwest in Utah to northwest in eastern Nevada, where it broadens to ???100 km width that correlates with early Paleozoic rifting of the continent. Our results suggest that the major gold belts may be over-isolated blocks of Archean crust, so Phanerozoic mineral deposits in this region may be produced, at least in part, from recycled Archean gold. Future mineral exploration to the east may yield large gold tonnages. ?? 2008 Geological Society of America.

  20. Kilbuck terrane: oldest known rocks in Alaska

    Box, S.E.; Moll-Stalcup, E. J.; Wooden, J.L.; Bradshaw, J.Y.

    1990-01-01

    The Kilbuck terrane in southwestern Alaska is a narrow, thin crustal sliver or flake of amphibolite facies orthogneiss. The igneous protolith of this gneiss was a suite of subduction-related plutonic rocks. U-Pb data on zircons from trondhjemitic and granitic samples yield upper-intercept (igneous) ages of 2070 ?? 16 and 2040 ?? 74 Ma, respectively. Nd isotope data from these rocks suggest that a diorite-tonalite-trondhjemite suite (??Nd[T] = +2.1 to +2.7; T is time of crystallization) evolved from partial melts of depleted mantle with no discernible contamination by older crust, whereas a coeval granitic pluton (??Nd[T] = -5.7) contains a significant component derived from Archean crust. Orthogneisses with similar age and Nd isotope characteristics are found in the Idono complex 250 km to the north. Early Proterozoic rocks are unknown elsewhere in Alaska. The possibility that the Kilbuck terrane was displaced from provinces of similar age in other cratons (e.g., Australian, Baltic, Guiana, and west African shields), or from the poorly dated Siberian craton, cannot be excluded. -from Authors

  1. Late Proterozoic island-arc complexes and tectonic belts in the southern part of the Arabian Shield, Kingdom of Saudi Arabia

    Greenwood, William R.; Stoeser, D.B.; Fleck, R.J.; Stacey, J.S.

    1983-01-01

    Sr ratios are not included in the appendix, but all rocks more than 660 m.y. old have initial ratios in the range 0.7021-0.7035, with only two greater than 0.7030. Thus, nothing in the Rb-Sr data suggests involvement of an older continental crust during the evolution of the southern Shield. A lead isotope study of ore minerals and potassium feldspars of the Arabian Shield by Stacey and others (1980) also suggests that no older (Archean to early Proterozoic) evolved continental-type crust underlies the southern Shield. An early summary of mapping (Schmidt and others, 1973) suggests that older sialic basement underlies the late Proterozoic layered rocks in the southern Shield. However, subsequent-mapping and the isotopic studies cited above have established that all of these rocks are of late Proterozoic age and that all rocks of the southern Shield that are more than 660 m.y. old have ensimatic or mantle isotopic characteristics. Figure 2 shows, with only two exceptions, that rocks more than 800 m.y. old are present west of the boundary separating the Tayyah and Khadra belts. The exceptions are two poorly controlled Rb-Sr ages obtained by Fleck (1980) on two quartz diorite plutons in the Malahah region (appendix 1, localities 26 and 27). Preliminary uranium-thorium zircon data of Stacey now suggest that one of these quartz diorite plutons (locality 26) has an age of approximately 640 m.y. Therefore, we prefer to discount the two dates of Fleck until further information is available. As noted earlier and as described below, most of the rocks of the southern Arabian Shield have characteristics typical of those formed in the island-arc environment by subduction-related processes. We shall refer to the group of rocks in the western part of the southern Shield, which formed from 1100 to 800 m.y. ago, as the 'older ensimatic-arc complex' and those in the eastern and northwestern parts, which formed from 800 to 690 m.y. ago, as the 'younger marginal-arc compl

  2. Distribution and diagenesis of microfossils from the lower Proterozoic Duck Creek Dolomite, Western Australia

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Strother, P. K.; Rossi, S.

    1988-01-01

    Two distinct generations of microfossils occur in silicified carbonates from a previously undescribed locality of the Lower Proterozoic Duck Creek Dolomite, Western Australia. The earlier generation occurs in discrete organic-rich clasts and clots characterized by microquartz anhedra; it contains a variety of filamentous and coccoidal fossils in varying states of preservation. Second generation microfossils consist almost exclusively of well-preserved Gunflintia minuta filaments that drape clasts or appear to float in clear chalcedony. These filaments appear to represent an ecologically distinct assemblage that colonized a substrate containing the partially degraded remains of the first generation community. The two assemblages differ significantly in taxonomic frequency distribution from previously described Duck Creek florules. Taken together, Duck Creek microfossils exhibit a range of assemblage variability comparable to that found in other Lower Proterozoic iron formations and ferruginous carbonates. With increasing severity of post-mortem alteration, Duck Creek microfossils appear to converge morphologically on assemblages of simple microstructures described from early Archean cherts. Two new species are described: Oscillatoriopsis majuscula and O. cuboides; the former is among the largest septate filamentous fossils described from any Proterozoic formation.

  3. The genesis of early Carboniferous adakitic rocks at the southern margin of the Alxa Block, North China

    NASA Astrophysics Data System (ADS)

    Xue, Shuo; Ling, Ming-Xing; Liu, Yu-Long; Zhang, Hong; Sun, Weidong

    2017-05-01

    Adakite is a highly debated petrologic term that was originally proposed to describe igneous rocks formed by slab melting. Subsequent studies reported other ways to generate adakitic signatures such as the melting of mafic lower continental crust and fractional crystallization of basaltic magma. We studied adakitic rocks from the Taohuala Mountain at the southern margin of the Alxa Block, North China. These rocks are characterized by high Sr concentrations (300-450 ppm), high Sr/Y (20-75 ppm) and (La/Yb)N (25-67 ppm) ratios, and low Y (< 18 ppm) and Yb (< 1.9 ppm) concentrations, which are typical of adakite. The distribution of these data on a Sr/Y versus (La/Yb)N discrimination diagram, combined with their high (87Sr/86Sr)i ratios (0.7113-0.7131) and low εNd(t) (- 15.8 to - 16.8) and εHf(t) (- 18 to - 10) values of zircon, indicates that the adakitic rocks formed by partial melting of thickened continental crust. U-Pb dating of zircons using LA-ICP-MS yields an early Carboniferous age of 330 ± 5 Ma. The ages and spatial distribution of magmatic rocks indicate that the Paleo-Asian oceanic crust subducted towards the Alxa Block in the late Paleozoic. Subsequently, northward slab rollback occurred during the Carboniferous. Therefore, we propose that the adakitic rocks from the Taohuala Mountain formed by partial melting of previously thickened lower continental crust, induced by the upwelling of asthenospheric mantle during slab rollback.

  4. Lead isotopic evidence for mixed sources of Proterozoic granites and pegmatites, Black Hills, South Dakota, USA

    NASA Astrophysics Data System (ADS)

    Krogstad, Eirik J.; Walker, Richard J.; Nabelek, Peter I.; Russ-Nabelek, Carol

    1993-10-01

    The lead isotopic compositions of K-feldspars separated from the ca. 1700 Ma Harney Peak Granite complex and spatially associated granitic pegmatites indicate that these rocks were derived from at least two sources. It has been reported previously that the core of the Harney Peak Granite complex is dominated by relatively lower/ gd18O (avg. 11.5 %.) granites, whereas higher / gd18O (avg. 13.2%.) granites occur around the periphery of the complex. The higher δ 18O granites and one simple pegmatite have low values of 207Pb /204Pb for their 206Pb /204Pb Thus, they likely were derived from a source with a short crustal residence time. This source may have been the pelitic schists into which the Harney Peak Granite complex and pegmatites were intruded. Feldspars from granites with lower / gd18O values have significantly higher 207Pb /204Pb for their 206Pb /204Pb . The data define a linear array with a slope equivalent to an age of ca. 2.6 Ga with t 2 defined to be 1.7 Ga. Such a slope could represent a mixing array or a secondary isochron for the source. These low δ18O granites could have been derived from a source with a high U/ Pb and with a crustal residence beginning before the Proterozoic. The source (s) of these granites may have been a sediment derived from late Archean continental crust. The highly evolved Tin Mountain pegmatite has lead isotopic systematics intermediate between those of the two granite groups, suggesting either a mixed source or contamination. Two late Archean granites, the Little Elk Granite and the Bear Mountain Granite, had precursors with high U/Pb and low Th/U histories. The Th/U history of the Bear Mountain Granite is too low for this rock to have been an important component of the source of the Proterozoic granites. However, crustal rocks with lead isotopic compositions similar to those of the Little Elk Granite were an important source of lead for some of the Proterozoic granitic rocks.

  5. Basement rocks of Halmahera, eastern Indonesia: Implications for the early history of the Philippine Sea

    SciT

    Hall, R.G.N.; Ballantyne, P.

    1990-06-01

    The oldest rocks known on Halmahera, eastern Indonesia, are petrologically and chemically similar to supra-subduction ophiolites and include boninitic volcanics resembling those dredged from the Marianas forearc. The age of the ophiolitic rocks is unknown; in east Halmahera they are overlain by Late Cretaceous and Eocene volcanics and associated sediments. Similar volcanics form the basement of western Halmahera. Plutonic rocks intruding the ophiolite and associated metamorphic rocks also yield Late Cretaceous to Eocene radiometric ages. The petrology and chemistry of the igneous rocks indicate an island arc origin. These rocks are locally overlain by shallow-water Eocene limestones and all aremore » overlain unconformably by Neogene sediments. The Halmahera basement rocks have many structural, petrological, and stratigraphic similarities to submarine plateaus of the southern and northern Philippine Sea and basement terranes of the eastern Philippines. The authors suggest that these similarities indicate the existence of an extensive region of Late Cretaceous and Eocene volcanism built upon probable Mesozoic ophiolitic basement. The resultant thickened crust was later fragmented by spreading in the West Philippine Sea Central Basin and backarc spreading in the Eastern Philippine Sea. It is difficult to reconcile the present distribution of these crustal fragments with a linear arc, but equally difficult to propose a simple alternative. A proto-Philippine archipelago, with short-lived arcs separated by small oceanic basins, may be the closest modern analog. The development of younger subduction zones has been influenced by the distribution of thickened crustal fragments as they have re-amalgamated since the Miocene.« less

  6. The Presence of a Stable Block bounded by Active Zones (Mobile Belts) in the southwestern North American Proterozoic craton

    NASA Astrophysics Data System (ADS)

    Goodell, P.; Martinez P, C.; Mahar, M. A.

    2014-12-01

    Bouguer gravity data, initial Sr isotope values, zircon U-Pb, and multiple occurrences of felsic Proterozoic rocks, have revealed an elevated, less deformed, felsic cratonic block in the northern Mexico. The block is situated in western Chihuahua and is bounded by active zones or mobile belts on three sides, and is here referred to as the Western Chihuahua Cratonic Block (WCCB). Bouguer gravity data clearly indicate a region of a highly negative anomaly (< -200 mgal) in contrast to adjoining areas. The region is large and the anomaly is relatively smooth over broad areas; the WCCB appears as a smaller version of the Colorado Plateau. The block is characterized by high initial Sr isotope ratios (<0.706). Several occurrences of Proterozoic rocks are located within or next to the WCCB, and they reveal the character of the Bouguer anomaly. On the east, at Los Filtros, Proterozoic rocks crop out in a basement cored uplift interpreted to having been derived from the WCCB during the Ouachita orogeny. At Sierra La Mojina boulders of 1.1 Ga granites are found in Permian conglomerates. And at Basasiachic, xenoliths of 1.1 Ga granites are present in ash flow tuffs. Establishment of the Precambrian character of the WCCB is of importance, and these multiple occurrences are evidence. Prior studies of the Sierra Madre Occidental suggest that the region was uplifted because of a vast Cenozoic batholith presumed to lie under the SLIP (Silicic Large Igneous Province), the Upper Volcanic Series. The present study challenges that conclusion and maintains the SMO is underlain by Proterozoic silicic crust. The geology of age dated samples supports this. The WCCB is surrounded on three sides by Active Zones or Mobile Belts, which have been active extensional and translational zones periodically over a long period of time. On the east are the Paleozoic Pedrogosa Basin, Mesozoic Chihuahua Trough and Cenozoic Rio Grande Rift, the first two of which also continue around the northern border

  7. Evolving Mantle Sources in Postcollisional Early Permian-Triassic Magmatic Rocks in the Heart of Tianshan Orogen (Western China)

    NASA Astrophysics Data System (ADS)

    Tang, Gong-Jian; Cawood, Peter A.; Wyman, Derek A.; Wang, Qiang; Zhao, Zhen-Hua

    2017-11-01

    Magmatism postdating the initiation of continental collision provides insight into the late stage evolution of orogenic belts including the composition of the contemporaneous underlying subcontinental mantle. The Awulale Mountains, in the heart of the Tianshan Orogen, display three types of postcollisional mafic magmatic rocks. (1) A medium to high K calc-alkaline mafic volcanic suite (˜280 Ma), which display low La/Yb ratios (2.2-11.8) and a wide range of ɛNd(t) values from +1.9 to +7.4. This suite of rocks was derived from melting of depleted metasomatized asthenospheric mantle followed by upper crustal contamination. (2) Mafic shoshonitic basalts (˜272 Ma), characterized by high La/Yb ratios (14.4-20.5) and more enriched isotope compositions (ɛNd(t) = +0.2 - +0.8). These rocks are considered to have been generated by melting of lithospheric mantle enriched by melts from the Tarim continental crust that was subducted beneath the Tianshan during final collisional suturing. (3) Mafic dikes (˜240 Ma), with geochemical and isotope compositions similiar to the ˜280 Ma basaltic rocks. This succession of postcollision mafic rock types suggests there were two stages of magma generation involving the sampling of different mantle sources. The first stage, which occurred in the early Permian, involved a shift from depleted asthenospheric sources to enriched lithospheric mantle. It was most likely triggered by the subduction of Tarim continental crust and thickening of the Tianshan lithospheric mantle. During the second stage, in the middle Triassic, there was a reversion to more asthenospheric sources, related to postcollision lithospheric thinning.

  8. Carboniferous - Early Permian magmatic evolution of the Bogda Range (Xinjiang, NW China): Implications for the Late Paleozoic accretionary tectonics of the SW Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Wali, Guzalnur; Wang, Bo; Cluzel, Dominique; Zhong, Linglin

    2018-03-01

    The Late Paleozoic magmatic evolution of the Bogda Range (Chinese North Tianshan) is important for understanding the accretionary history of the Central Asian Orogenic Belt. We investigated the Carboniferous and Lower Permian volcanic and sedimentary sequences of the Daheyan section, southern Bogda Range, and present new zircon U-Pb ages and whole-rock geochemical data for the volcanic rocks. One Carboniferous rhyolite is dated at 298 ± 8 Ma; a Permian basalt yielded many Proterozoic zircon xenocrysts, and its maximum age (∼297 Ma) is constrained by the detrital zircon ages of the sandstone that stratigraphically underlies it. These volcanic rocks belong to calc-alkaline series. We further synthesize previous geochronological, geochemical and isotopic data of magmatic and sedimentary rocks in the Bogda Range. The available data indicate that the magmatism occurred continuously from 350 Ma to 280 Ma. A comprehensive analysis allows us to propose that: (1) the Carboniferous to Early Permian magmatic rocks of the Bogda Range generally show consistent arc-type features; (2) increasing mantle input through time suggests intra-arc extension in a supra-subduction zone; (3) the localized occurrence of Early Permian alkaline pillow basalts and deep water sediments close to the major shear zone advocate a transtensional crustal thinning during the transition from Carboniferous convergence to Early Permian transcurrent tectonics; (4) occurrence of a large number of Proterozoic zircon xenocrysts in the Late Paleozoic magmatic rocks, and Proterozoic detrital zircons in the coeval clastic sediments suggest a continental or transitional basement of the Bogda Arc; (5) subduction in the Bogda area terminated prior to the deposition of Middle Permian terrestrial sediments.

  9. Excess lead in "rusty rock" 66095 and implications for an early lunar differentiation

    Nunes, P.D.; Tatsumoto, M.

    1973-01-01

    Apollo 16 breccia 66095 contains a remarkably high amount of lead (15 part's per million), 85 percent of which is not supported by uranium and thorium in the rock. An acid leach experiment coupled with separate analyses of the whole rock and mineral fractions for uranium, thorium, and lead indicate that the excess lead has a lunar source and was apparently introduced about 4.0 X 109 years ago. The data also suggest that a major lunar crustal differentiation occurred about 4.47 X 109 years ago.

  10. Petrogenesis and tectonic implications of Early Cretaceous volcanic rocks from Lingshan Island in the Sulu Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Meng, Yuanku; Santosh, M.; Li, Rihui; Xu, Yang; Hou, Fanghui

    2018-07-01

    The Dabie-Sulu orogenic belt in eastern China marks the boundary between the Yangtze Block and the North China Block. Here we investigate a suite of volcanic rocks from Lingshan Island in the Sulu belt comprising rhyolite, trachyte, trachyandesite and basaltic trachyandesite. We present petrological, geochemical and zircon Usbnd Pb ages and Hfsbnd O isotope data with a view to gain insights on the petrogenesis and tectonic implications. SHRIMP II analyses of zircon grains from the rhyolite yield 206Pb/238U age of 127.6 ± 1.3 Ma and LA-MC-ICP-MS dating show 126.3 ± 1.2 Ma and 127.3 ± 1.1 Ma, together constraining the eruption time as Early Cretaceous. LA-MC-ICP-MS analyses of zircon grains from the andesitic rocks yield 206Pb/238U ages of 129.0 ± 1.6 Ma, 129.8 ± 1.5 Ma and 130.9 ± 1.0 Ma. Geochemically, the rhyolite shows shoshonitic features with low MgO and Cr, but high Na2O + K2O. The zircon grains from these rocks yield negative εHf(t) values and low δ18O values, and these together with the presence of Neoproterozoic inherited zircons suggest that the magma source involved melting of the Yangtze crust. The andesitic rocks, including basaltic trachyandesite, trachyandesite and trachyte, show a wide range of SiO2, Mg# values, and Cr, enriched in LILE and LREE, depleted in HFSE (Nb, Ta and Ti), and have significantly negative zircon εHf(t) values, suggesting derivation from subcontinental lithosphere mantle that was metasomatized by felsic melts. Our results, integrated with those from previous studies suggest heterogeneous magma involving the mixing of mantle and crustal sources within an extensional setting in the Early Cretaceous.

  11. Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen

    NASA Technical Reports Server (NTRS)

    Swett, K.; Knoll, A. H.

    1989-01-01

    Upper Proterozoic carbonate successions from central East Greenland (the Limestone-Dolomite 'Series' of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen) Group, Spitsbergen, and the Upper Russo Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine environments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake-conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross-beds and megaripples that characterize the pisolitic beds; and by microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio-, litho-, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres.

  12. Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen.

    PubMed

    Swett, K; Knoll, A H

    1989-01-01

    Upper Proterozoic carbonate successions from central East Greenland (the Limestone-Dolomite 'Series' of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen) Group, Spitsbergen, and the Upper Russo Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine environments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake-conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross-beds and megaripples that characterize the pisolitic beds; and by microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio-, litho-, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres.

  13. Rock Music and the Socialization of Moral Values in Early Adolescence.

    ERIC Educational Resources Information Center

    Leming, James S.

    1987-01-01

    Links between rock music, moral values, and youth behavior are difficult to establish. This study shows that while young people are influenced by the content of songs, they sometimes disagree with and criticize lyrics. Thus the premise of adolescents as passive receptors of negative values portrayed in music is not warranted. (VM)

  14. Morphometric convergence between Proterozoic and post-vegetation rivers

    PubMed Central

    Ielpi, Alessandro; Rainbird, Robert H.; Ventra, Dario; Ghinassi, Massimiliano

    2017-01-01

    Proterozoic rivers flowed through barren landscapes, and lacked interactions with macroscopic organisms. It is widely held that, in the absence of vegetation, fluvial systems featured barely entrenched channels that promptly widened over floodplains during floods. This hypothesis has never been tested because of an enduring lack of Precambrian fluvial-channel morphometric data. Here we show, through remote sensing and outcrop sedimentology, that deep rivers were developed in the Proterozoic, and that morphometric parameters for large fluvial channels might have remained within a narrow range over almost 2 billion years. Our data set comprises fluvial-channel forms deposited a few tens to thousands of kilometres from their headwaters, likely the record of basin- to craton-scale systems. Large Proterozoic channel forms present width:thickness ranges matching those of Phanerozoic counterparts, suggesting closer parallels between their fluvial dynamics. This outcome may better inform analyses of extraterrestrial planetary surfaces and related comparisons with pre-vegetation Earth landscapes. PMID:28548109

  15. Morphometric convergence between Proterozoic and post-vegetation rivers.

    PubMed

    Ielpi, Alessandro; Rainbird, Robert H; Ventra, Dario; Ghinassi, Massimiliano

    2017-05-26

    Proterozoic rivers flowed through barren landscapes, and lacked interactions with macroscopic organisms. It is widely held that, in the absence of vegetation, fluvial systems featured barely entrenched channels that promptly widened over floodplains during floods. This hypothesis has never been tested because of an enduring lack of Precambrian fluvial-channel morphometric data. Here we show, through remote sensing and outcrop sedimentology, that deep rivers were developed in the Proterozoic, and that morphometric parameters for large fluvial channels might have remained within a narrow range over almost 2 billion years. Our data set comprises fluvial-channel forms deposited a few tens to thousands of kilometres from their headwaters, likely the record of basin- to craton-scale systems. Large Proterozoic channel forms present width:thickness ranges matching those of Phanerozoic counterparts, suggesting closer parallels between their fluvial dynamics. This outcome may better inform analyses of extraterrestrial planetary surfaces and related comparisons with pre-vegetation Earth landscapes.

  16. Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

    Amato, J.M.; Toro, J.; Miller, E.L.; Gehrels, G.E.; Farmer, G.L.; Gottlieb, E.S.; Till, A.B.

    2009-01-01

    The Seward Peninsula of northwestern Alaska is part of the Arctic Alaska-Chukotka terrane, a crustal fragment exotic to western Laurentia with an uncertain origin and pre-Mesozoic evolution. U-Pb zircon geochronology on deformed igneous rocks reveals a previously unknown intermediate-felsic volcanic event at 870 Ma, coeval with rift-related magmatism associated with early breakup of eastern Rodinia. Orthogneiss bodies on Seward Peninsula yielded numerous 680 Ma U-Pb ages. The Arctic Alaska-Chukotka terrane has pre-Neoproterozoic basement based on Mesoproterozoic Nd model ages from both 870 Ma and 680 Ma igneous rocks, and detrital zircon ages between 2.0 and 1.0 Ga in overlying cover rocks. Small-volume magmatism occurred in Devonian time, based on U-Pb dating of granitic rocks. U-Pb dating of detrital zircons in 12 samples of metamorphosed Paleozoic siliciclastic cover rocks to this basement indicates that the dominant zircon age populations in the 934 zircons analyzed are found in the range 700-540 Ma, with prominent peaks at 720-660 Ma, 620-590 Ma, 560-510 Ma, 485 Ma, and 440-400 Ma. Devonian- and Pennsylvanian-age peaks are present in the samples with the youngest detrital zircons. These data show that the Seward Peninsula is exotic to western Laurentia because of the abundance of Neoproterozoic detrital zircons, which are rare or absent in Lower Paleozoic Cordilleran continental shelf rocks. Maximum depositional ages inferred from the youngest detrital age peaks include latest Proterozoic-Early Cambrian, Cambrian, Ordovician, Silurian, Devonian, and Pennsylvanian. These maximum depositional ages overlap with conodont ages reported from fossiliferous carbonate rocks on Seward Peninsula. The distinctive features of the Arctic Alaska-Chukotka terrane include Neoproterozoic felsic magmatic rocks intruding 2.0-1.1 Ga crust overlain by Paleozoic carbonate rocks and Paleozoic siliciclastic rocks with Neoproterozoic detrital zircons. The Neoproterozoic ages are

  17. Ring complexes and related rocks in Africa

    NASA Astrophysics Data System (ADS)

    Vail, J. R.

    Over 625 igneous complexes throughout Africa and Arabia have been selected and classified on the basis of petrographic association and chronology into six broad age groups forming 29 provinces. The groups range from Mid-Proterozoic to Tertiary and include gabbro, granite, syenite, foid syenite and carbonatite plutonic rocks, the majority in the form of ring-dykes, cone-sheets, plugs, circular intrusions, and their associated extrusive phases. Pan-African late or post-orogenic complexes (720-490 Ma) are common in the Arabian-Nubian and Tuareg shields of north Africa originating from subduction zone derived magmatism. Anorogenic complexes in Egypt, NE and central Sudan, Niger, Nigeria, Cameroon, Zaïre-Burundi, Malawi, Mozambique, Zimbabwe, Namibia and Angola span 550 to 50 Ma and are dominantly alkali granites and foid syenites. Many groups occur as en-echelon bands within linear arrays, and show migrating centres of intrusion in variable directions. In W. Africa there was a progressive shift of emplacement southwards during early Ordovician to Mid-Cretaceous times. Distribution patterns suggest thatdeep seated features, such as shear zones associated with lithospheric plate movements,controlled melting, and the resultant location of the complexes. Economic mineralization is not widespread in the rocks of the African ring complexes and is mainly restricted to small deposits of Sn, W, F, U and Nb.

  18. Mineralogy of Rock Flour in Glaciated Volcanic Terrains: An Analog for a Cold and Icy Early Mars

    NASA Technical Reports Server (NTRS)

    Rampe, E. B.; Horgan, B.; Scudder, N.; Smith, R. J.; Rutledge, A. M.

    2017-01-01

    Geomorphological and mineralogical data from early Martian surfaces indicate liquid water was present on ancient Mars. The relative surface temperatures, however, remain a subject of debate. Was early Mars warm and wet or cold and icy with punctuated periods of warmth and ice melt? By characterizing the mineralogy and geochemistry of modern icy mafic terrains on Earth, we can search for these characteristics in early Martian terrains to better constrain the early Martian climate. Here, we describe the mineralogy of glacial flour in a modern glaciated volcanic terrain in Oregon, USA. We are particularly interested in secondary phases that form in these environments, and we hypothesize that poorly crystalline phases may preferentially form in these terrains because of the low temperatures and the seasonality of melt water production. A description of the mineralogy of the moraines, the composition of the amorphous materials, and the geochemistry of the glacial melt waters are presented elsewhere. Glacial flour is made up of silt- and clay-sized particles that form from the physical weathering of rock underlying a wet-based glacier as the glacier slides over it. Flour is usually transported from underneath a glacier by melt water streams. The geochemistry of glacial melt water streams has been studied extensively and has been used to infer weathering reactions within glacial systems. However, the mineralogy of these environments, especially on mafic volcanic terrains, is not well studied. Rock flour is a ubiquitous physical weathering product in glaciated terrains and, therefore, affects microbial habitats, stream and lake chemistry, and chemical weathering processes. and by studying the mineralogy of glacial flour, we can better understand geochemical and microbiological processes in subglacial and proglacial terrains.

  19. Trace sulfate in mid-Proterozoic carbonates and the sulfur isotope record of biospheric evolution

    NASA Astrophysics Data System (ADS)

    Gellatly, Anne M.; Lyons, Timothy W.

    2005-08-01

    Concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the Earth's surface. This balance is expressed in the sulfur isotope composition of seawater sulfate. Given the near absence of gypsum in pre-Phanerozoic sediments, trace amounts of carbonate-associated sulfate (CAS) within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in Precambrian seawater. Although absolute CAS concentrations, which range from those below detection to ˜120 ppm sulfate in this study, may be compromised by diagenesis, the sulfur isotope compositions can be buffered sufficiently to retain primary values. Stratigraphically controlled δ 34S measurements for CAS from three mid-Proterozoic carbonate successions (˜1.2 Ga Mescal Limestone, Apache Group, Arizona, USA; ˜1.45-1.47 Ga Helena and Newland formations, Belt Supergroup, Montana, USA; and ˜1.65 Ga Paradise Creek Formation, McNamara Group, NW Queensland, Australia) show large isotopic variability (+9.1‰ to +18.9‰, -1.1‰ to +27.3‰, and +14.1‰ to +37.3‰, respectively) over stratigraphic intervals of ˜50 to 450 m. This rapid variability, ranging from scattered to highly systematic, and overall low CAS abundances can be linked to sulfate concentrations in the mid-Proterozoic ocean that were substantially lower than those of the Phanerozoic but higher than values inferred for the Archean. Results from the Belt Supergroup specifically corroborate previous arguments for seawater contributions to the basin. Limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep-ocean sulfate reduction, the scarcity of bedded gypsum, and the stratigraphic δ 34S trends and 34S enrichments commonly observed for iron sulfides of mid-Proterozoic age.

  20. Variation in 142Nd/144Nd of Archean rocks from southwest Greenland : Implications for early Earth mantle dynamics

    NASA Astrophysics Data System (ADS)

    Rizo, H.; Boyet, M.; Blichert-Toft, J.; Rosing, M.; Paquette, J. L.

    2012-04-01

    The short-lived 146Sm-142Nd chronometer (half-life = 103 Ma) has proven successful in bringing constraints on the dynamics of the early Earth mantle. Since the parent isotope, 146Sm, was extant only during the first 300 Ma of the history of the Solar System, the positive 142Nd anomalies measured in southwest Greenland Archean rocks imply that their incompatible element-depleted mantle source formed during the Hadean. Interestingly, the magnitude of these anomalies seems to decrease over time. 3.7-3.8 Ga old rocks from the Amitsoq Complex have revealed +10 to +20 ppm 142Nd anomalies [1, 2, 3, 4, 5, 6, 7], whereas younger 3.0 Ga old samples from the Ivisaartoq greenstone belt yield smaller positive anomalies, ranging from +5.5 to +8.5 ppm [8]. Thus, the chemical heterogeneities detected in the southwest Greenland mantle were formed during the first 150 Ma of Earth's history, and seem to have resisted re-mixing by mantle convection until 3.0 Ga. In this study, we investigate the evolution of the southwest Greenland mantle during the time period of 3.3-3.4 Ga. The samples analyzed come from both the ~3.3 Ga amphibolite unit and the ~3.4 Ga Ameralik basic dyke swarm from the Amitsoq Complex. Coupled Sm-Nd and Lu-Hf bulk-rock ages obtained for seven amphibolites are in good agreement (3351 ± 210 Ma and 3302 ± 260 Ma, respectively) and consistent with the minimum age found by Nutman and Friend (2009) [9] for this formation. We further obtained coherent bulk-rock 147Sm-143Nd and zircon+baddeleyite 207Pb/206Pb ages for the Ameralik dykes (3428 ± 250 Ma and 3421 ± 34 Ma, respectively), in line with ages suggested by Nielsen at al., (2002) [10] and Nutman et al., (2004) [11]. We are currently in the process of analyzing these samples for 142Nd isotopic compositions and the results will be compared with the existing southwest Greenland data in order to shed new light on the evolution and destruction of heterogeneities in the early Earth mantle. [1] Rizo et al., (2011

  1. Prokaryotic algae associated with Australian proterozoic stromatolites.

    PubMed

    Licari, G R; Cloud, P

    1972-09-01

    Five instances of association between distinctive stromatolites and blue-green algal nannofossils are recorded from a 100-m sequence of carbonate rocks about 1.6 x 10(9) years old, along the south side of Paradise Creek, northwestern Queensland, Australia. No eukaryotes were identified in any of these systematically limited assemblages, although they are known from rocks as old as 1.3 x 10(9) years in eastern California. Thus, eukaryotes may not have appeared until after 1.6 x 10(9) years ago (but before 1.3 x 10(9) years ago). The associations observed would also be consistent with (but do not prove) a biotic influence on stromatolite morphology. As is usual among prePaleozoic forms described, the morphology of the nannofossils is very similar to living forms, displaying marked evolutionary conservatism. Primary orientation of stromatolitic laminae and columns is not invariably convex upward, as conventionally believed, but convex away from and parallel to the initial point or surface of attachment, which may be horizontal or even downward beneath overhangs.

  2. Paleomagnetism of Early Paleozoic Rocks from the de Long Archipelago and Tectonics of the New Siberian Islands Terrane

    NASA Astrophysics Data System (ADS)

    Metelkin, D. V.; Chernova, A. I.; Matushkin, N. Y.; Vernikovskiy, V. A.

    2017-12-01

    The De Long archipelago is located to the north of the Anjou archipelago as a part of a large group between the Laptev Sea and the East Siberian Sea - the New Siberian Islands and consists of Jeannette Island, Bennett Island and Henrietta Island. These islands have been shown to be part of a single continental terrane, whose tectonic history was independent of other continental masses at least since the Ordovician. Paleomagnetic and precise geological data for the De Long archipelago were absent until recently. Only in 2013 special international field trips to the De Long Islands could be organized and geological, isotope-geochronological and paleomagnetic studies were carried out.On Jeannette Island a volcanic-sedimentary sequence intruded by mafic dikes was described. The age of these dikes is more likely Early Ordovician, close to 480 Ma, as evidenced by the results of our 40Ar/39Ar and paleomagnetic investigations of the dolerites as well as the result from detrital zircons in the host rocks published before. On Bennett Island, there are widespread Cambrian-Ordovician mainly terrigenous rocks. Paleomagnetic results from these rocks characterize the paleogeographic position of the De Long archipelago at 465 Ma and perhaps at 530 Ma, although there is no evidence for the primary origin of magnetization for the latter. On Henrietta Island the Early Cambrian volcanic-sedimentary section was investigated. A paleomagnetic pole for 520 Ma was obtained and confirmed by new 40Ar/39Ar results. Adding to our previous paleomagnetic data for the Anjou archipelago the extended variant of the apparent polar wander path for the New Siberian Island terrane was created. The established paleolatitudes define its location in the equatorial and subtropical zone no higher than 40 degrees during the Early Paleozoic. Because there are no good confirmations for true polarity and related geographic hemisphere we present two possibilities for tectonic reconstruction. But both these

  3. Carbon, nitrogen, and sulfur geochemistry of Archean and Proterozoic shales from the Kaapvaal Craton, South Africa

    NASA Astrophysics Data System (ADS)

    Watanabe, Yumiko; Naraoka, Hiroshi; Wronkiewicz, David J.; Condie, Kent C.; Ohmoto, Hiroshi

    1997-08-01

    The C, N, and S contents and VC and δ 13Cδ 34S values were analyzed for 100 shale samples from ten formations, 3.0 to 2.1 Ga in age, in the central and eastern regions of the Kaapvaal Craton, South Africa. The Kaapvaal shales are characterized by generally low contents of organic C (range 0.06-2.79 wt%, average 0.47 wt%), N (range <0.01-0.09 wt%, average 0.1 wt%), and S (range <0.01-1.63 wt%, average 0.1 wt%). The low N/C (<0.005) and H/C (mostly ˜0.2) atomic ratios in kerogens from the shales indicated that the Kaapvaal shales lost considerable amounts of N, C, S, and H during diagenesis and regional metamorphism (up to the greenschist facies). From the theoretical relationships between the H/C ratios of kerogen and organic C contents of shales, the original C contents of the Archean and Proterozoic shales from the Kaapvaal Craton are estimated to be on average ˜2 wt%. These values are similar to the average organic C content of modern marine sediments. This suggests that the primary organic productivity and the preservation of organic matter in the ocean during the period of 3.0 to 2.1 Ga were similar to those in the Phanerozoic era, provided the flux of clastic sediments to the ocean was similar. This would also imply that the rate of O 2 accumulation in the atmosphere-ocean system, which has equaled the burial rate of organic matter in sediments, has been the same since ˜3.0 Ga. The δ 34S values of bulk-rock sulfides (mostly pyrite) range from +2.7 to +7.4%‰ for seven sulfide-rich samples of ˜2.9 Ga to ˜2.6 Ga. These values are consistent with a suggestion by Ohmoto (1992) and Ohmoto et al. (1993) that most pyrite crystals in Archean shales were formed by bacterial reduction of seawater sulfate with δ 34S values between +2 and +10‰, and that the Archean seawater was sulfate rich. Changes in the δ 13C org values during maturation of kerogen were evaluated with theoretical calculations from the experimental data of Peters et al. (1981) and Lewan

  4. Constraining the redox landscape of the mid-Proterozoic oceans: new insights from the carbonate uranium isotope record

    NASA Astrophysics Data System (ADS)

    Gilleaudeau, G. J.; Kaufman, A. J.; Luo, G.; Romaniello, S. J.; Zhang, F.; Kah, L. C.; Azmy, K.; Bartley, J. K.; Sahoo, S. K.; Knoll, A. H.; Anbar, A. D.

    2017-12-01

    The redox landscape of the global oceans during the prolonged period between the Great Oxidation Event (GOE) and the Neoproterozoic Oxygenation Event (NOE) is a topic of considerable debate. Data from local redox proxies such as iron speciation suggest largely ferruginous conditions in the subsurface oceans (with the exception of one report of oxic subsurface waters) and a variable degree of euxinia in shallow shelf and epeiric sea environments. There is general consensus that anoxia was more widespread than in the modern ocean, but quantifying the degree of seafloor anoxia is challenging given that most redox proxies are inherently local and/or based on the relatively sparse black shale record. Here, we present new uranium (U) isotope data from carbonate rocks than span the mid-Proterozoic Eon. U-isotopes operate as a proxy for seafloor anoxia because the δ238U value of seawater is largely controlled by the size of the anoxic/euxinic U sink, which preferentially removes isotopically heavy 238U, leaving the oceans enriched in 235U. Our compilation of data from mid-Proterozoic successions reveals δ238U values similar to modern seawater (-0.39 ± 0.19 ‰ [1 s.d.] for the Gaoyuzhuang, Angmaat, El Mreiti, Vazante, and Turukhansk successions spanning 1.5 to 0.9 Ga). Given the potential for an isotopic offset between carbonate minerals and seawater of up to 0.3 ‰, we suggest that mid-Proterozoic seawater had a δ238U value generally between -0.4 and -0.7 ‰, which is lower than modern seawater, but higher than has been inferred for intervals of expanded anoxia elsewhere in Earth history. These results are consistent with recently published U-isotope data from the 1.36 Ga Velkerri Formation, and suggest that large portions of the seafloor may have been covered by at least weakly oxygenated waters during the mid-Proterozoic Eon. Uncertainty remains, however, because the isotopic effects of the non-euxinic anoxic sink are poorly constrained. Nonetheless, our data

  5. Modern and ancient geochemical constraints on Proterozoic atmosphere-ocean redox evolution

    NASA Astrophysics Data System (ADS)

    Hardisty, D. S.; Horner, T. J.; Wankel, S. D.; Lu, Z.; Lyons, T.; Nielsen, S.

    2017-12-01

    A detailed understanding of the spatiotemporal oxygenation of Earth's atmosphere-ocean system through the Precambrian has important implications for the environments capable of sustaining early eukaryotic life and the evolving oxidant budget of subducted sediments. Proxy records suggest an anoxic Fe-rich deep ocean through much of the Precambrian and atmospheric and surface-ocean oxygenation that started in earnest at the Paleoproterozoic Great Oxidation Event (GOE). The marine photic zone represented the initial site of oxygen production and accumulation via cyanobacteria, yet our understanding of surface-ocean oxygen contents and the extent and timing of oxygen propagation and exchange between the atmosphere and deeper ocean are limited. Here, we present an updated perspective of the constraints on atmospheric, surface-ocean, and deep-ocean oxygen contents starting at the GOE. Our research uses the iodine content of Proterozoic carbonates as a tracer of dissolved iodate in the shallow ocean, a redox-sensitive species quantitatively reduced in modern oxygen minimum zones. We supplement our understanding of the ancient record with novel experiments examining the rates of iodate production from oxygenated marine environments based on seawater incubations. Combining new data from iodine with published shallow marine (Ce anomaly, N isotopes) and atmospheric redox proxies, we provide an integrated view of the vertical redox structure of the atmosphere and ocean across the Proterozoic.

  6. Classic Rock

    ERIC Educational Resources Information Center

    Beem, Edgar Allen

    2004-01-01

    While "early college" programs designed for high-school-age students are beginning to proliferate nationwide, a small New England school has been successfully educating teens for nearly four decades. In this article, the author features Simon's Rock, a small liberal arts college located in the Great Barrington, Massachusetts, that has…

  7. Significance of tourmaline-rich rocks in the north range group of the cuyuna iron range, East-Central Minnesota

    Cleland, J.M.; Morey, G.B.; McSwiggen, P.L.

    1996-01-01

    Concentrations of tourmaline in Early Proterozoic metasedirnentary rocks of the Cuyuna iron range, east-central Minnesota, provide a basis for redefinition of the evolutionary history of the area. Manganiferous iron ore forms beds within the Early Proterozoic Trommald Formation, between thick-bedded granular iron-formation having shallow-water depositional attributes and thin-bedded, nongranular iron-formation having deeper water attributes. These manganese-rich units were previously assumed to be sedimentary in origin. However, a revaluation of drill core and mine samples from the Cuyuna North range has identified strata-bound tourmaline and tourmalinite, which has led to a rethinking of genetic models for the geology of the North range. We interpret the tourmaline-rich rocks of the area to be a product of submarine-hydrothermal solutions flowing along and beneath the sedirnent-seawater interface. This model for the depositional environment of the tourmaline is supported by previously reported mineral assemblages within the Trommald Formation that comprise aegirine; barium feldspar; manganese silicates, carbonates, and oxides; and Sr-rich barite veins. In many places, tourmaline-rich metasedimentary rocks and tourmalinites are associated locally with strata-bound sulfide deposits. At those localities, the tourmaline-rich strata are thought to be lateral equivalents of exhalative sulfide zones or genetically related subsea-floor replacements. On the basis of the occurrence of the tourmaline-rich rocks and tourmalinites, and on the associated minerals, we suggest that there is a previously unrecognized potential for sediment-hosted sulfide deposits in the Cuyuna North range.

  8. Coupled Nd-142, Nd-143 and Hf-176 Isotopic Data from 3.6-3.9 Ga Rocks: New Constraints on the Timing of Early Terrestrial Chemical Reservoirs

    NASA Technical Reports Server (NTRS)

    Bennett, Vickie C.; Brandon, alan D.; Hiess, Joe; Nutman, Allen P.

    2007-01-01

    Increasingly precise data from a range of isotopic decay schemes, including now extinct parent isotopes, from samples of the Earth, Mars, Moon and meteorites are rapidly revising our views of early planetary differentiation. Recognising Nd-142 isotopic variations in terrestrial rocks (which can only arise from events occurring during the lifetime of now extinct Sm-146 [t(sub 1/2)=103 myr]) has been an on-going quest starting with Harper and Jacobsen. The significance of Nd-142 variations is that they unequivocally reflect early silicate differentiation processes operating in the first 500 myr of Earth history, the key time period between accretion and the beginning of the rock record. The recent establishment of the existence of Nd-142 variations in ancient Earth materials has opened a new range of questions including, how widespread is the evidence of early differentiation, how do Nd-142 compositions vary with time, rock type and geographic setting, and, combined with other types of isotopic and geochemical data, what can Nd-142 isotopic variations reveal about the timing and mechanisms of early terrestrial differentiation? To explore these questions we are determining high precision Nd-142, Nd-143 and Hf-176 isotopic compositions from the oldest well preserved (3.63- 3.87 Ga), rock suites from the extensive early Archean terranes of southwest Greenland and western Australia.

  9. Stable isotope, chemical, and mineral compositions of the Middle Proterozoic Lijiaying Mn deposit, Shaanxi Province, China

    Yeh, Hsueh-Wen; Hein, James R.; Ye, Jie; Fan, Delian

    1999-01-01

    The Lijiaying Mn deposit, located about 250 km southwest of Xian, is a high-quality ore characterized by low P and Fe contents and a mean Mn content of about 23%. The ore deposit occurs in shallow-water marine sedimentary rocks of probable Middle Proterozoic age. Carbonate minerals in the ore deposit include kutnahorite, calcite, Mn calcite, and Mg calcite. Carbon (−0.4 to −4.0‰) and oxygen (−3.7 to −12.9‰) isotopes show that, with a few exceptions, those carbonate minerals are not pristine low-temperature marine precipitates. All samples are depleted in rare earth elements (REEs) relative to shale and have negative Eu and positive Ce anomalies on chondrite-normalized plots. The Fe/Mn ratios of representative ore samples range from about 0.034 to <0.008 and P/Mn from 0.0023 to <0.001. Based on mineralogical data, the low ends of those ranges of ratios are probably close to ratios for the pure Mn minerals. Manganese contents have a strong positive correlation with Ce anomaly values and a moderate correlation with total REE contents. Compositional data indicate that kutnahorite is a metamorphic mineral and that most calcites formed as low-temperature marine carbonates that were subsequently metamorphosed. The braunite ore precursor mineral was probably a Mn oxyhydroxide, similar to those that formed on the deep ocean-floor during the Cenozoic. Because the Lijiaying precursor mineral formed in a shallow-water marine environment, the atmospheric oxygen content during the Middle Proterozoic may have been lower than it has been during the Cenozoic.

  10. Ultra-high precision 142Nd/144Nd measurements of the Proterozoic and implications for mixing in the Earth's mantle through time

    NASA Astrophysics Data System (ADS)

    Hyung, E.; Jacobsen, S. B.

    2017-12-01

    The decay of 146Sm to 142Nd is an excellent a tracer for early silicate differentiation events in the terrestrial planets, as the Sm/Nd ratio is usually fractionated during mantle partial melting and magma ocean crystallization. The short half-life (103 or 68 Ma) renders the system extinct within the first 500 Ma of Solar System formation. Samples with 142Nd/144Nd ratios that are substantially different from the bulk silicate Earth value of 142Nd/144Nd provide clear evidence for mantle differentiation in the Hadean. Published data for the 3.4 to 3.8 Ga old Isua supracrustal rocks and dykes have demonstrated both positive and negative 142Nd/144Nd anomalies (30 ppm range) providing clear evidence for Hadean enriched and depleted mantle reservoirs. In contrast, no 142Nd/144Nd anomalies have been found in modern day terrestrial samples with data that have 2σ uncertainties of about 5 ppm or more. Last year we reported improvements in 142Nd/144Nd measurements, using our IsotopX thermal ionization mass spectrometer, and obtained reproducibility of 142Nd/144Nd ratios to better than 2 ppm at the 2σ level. With this external reproducibility we found that all except one modern mantle-derived basalt had within error identical 142Nd/144Nd ratios. One sample is about 3.4 ppm lower than the rest of the modern basalt samples, providing evidence for some limited Hadean mantle differentiation signatures preserved up to present. We have also measured 142Nd/144Nd ratios for Proterozoic and Phanerozoic samples, whose ages range from 300 Ma to 2 Ga, to better than 2 ppm external reproducibility (2σ). Most of these samples also have 142Nd/144Nd ratios that cluster around the modern day value, but there are some samples that are either marginally high by 2 ppm or low by 2 ppm. Thus, while a 20 to 30 ppm range in 142Nd/144Nd is well resolved in the Archean, such large variability is not present in the Proterozoic and Phanerozoic. The relatively rapid changeover at the end of the Archean

  11. Evidence for extreme mantle fractionation in early Archaean ultramafic rocks from northern Labrador

    NASA Technical Reports Server (NTRS)

    Collerson, Kenneth D.; Campbell, Lisa M.; Weaver, Barry L.; Palacz, Zenon A.

    1991-01-01

    Samarium-neodymium isotope data for tectonically interleaved fragments of lithospheric mantle and meta-komatiite from the North Atlantic craton provide the first direct record of mantle differentiation before 3,800 Myr ago. The results confirm the magnitude of light-rare-earth-element depletion in the early mantle, and also its depleted neodymium isotope composition. The mantle fragments were able to retain these ancient geochemical signatures by virtue of having been tectonically incorporated in buoyant felsic crust, thus escaping recycling and homogenization by mantle convection.

  12. Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

    Cox, R.; Coleman, D.S.; Chokel, C.B.; DeOreo, S.B.; Wooden, Joseph L.; Collins, A.S.; De Waele, B.; Kroner, A.

    2004-01-01

    Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

  13. Compositional controls on early diagenetic pathways in fine-grained sedimentary rocks: Implications for predicting unconventional reservoir attributes of mudstones

    Keller, Margaret A.; Macquaker, Joe H.S.; Taylor, Kevin G.; Polya, David

    2014-01-01

    Diagenesis significantly impacts mudstone lithofacies. Processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks. Selected organic-carbon-rich mudstones, from the Kimmeridge Clay and Monterey Formations, have been investigated to determine how varying starting compositions influence diagenesis.The sampled Kimmeridge Clay Formation mudstones are organized into thin homogenous beds, composed mainly of siliciclastic detritus, with some constituents derived from water-column production (e.g., coccoliths, S-depleted type-II kerogen, as much as 52.6% total organic carbon [TOC]) and others from diagenesis (e.g., pyrite, carbonate, and kaolinite). The sampled Monterey Formation mudstones are organized into thin beds that exhibit pelleted wavy lamination, and are predominantly composed of production-derived components including diatoms, coccoliths, and foraminifera, in addition to type-IIS kerogen (as much as 16.5% TOC), and apatite and silica cements.During early burial of the studied Kimmeridge Clay Formation mudstones, the availability of detrital Fe(III) and reactive clay minerals caused carbonate- and silicate-buffering reactions to operate effectively and the pore waters to be Fe(II) rich. These conditions led to pyrite, iron-poor carbonates, and kaolinite cements precipitating, preserved organic carbon being S-depleted, and sweet hydrocarbons being generated. In contrast, during the diagenesis of the sampled Monterey Formation mudstones, sulfide oxidation, coupled with opal dissolution and the reduced availability of both Fe(III) and reactive siliciclastic detritus, meant that the pore waters were poorly buffered and locally acidic. These conditions resulted in local carbonate dissolution, apatite and silica cements precipitation, natural kerogen sulfurization, and sour hydrocarbons generation.Differences in mud composition at deposition significantly influence subsequent

  14. Microscale mapping of alteration conditions and potential biosignatures in basaltic-ultramafic rocks on early Earth and beyond.

    PubMed

    Grosch, Eugene G; McLoughlin, Nicola; Lanari, Pierre; Erambert, Muriel; Vidal, Olivier

    2014-03-01

    Subseafloor environments preserved in Archean greenstone belts provide an analogue for investigating potential subsurface habitats on Mars. The c. 3.5-3.4 Ga pillow lava metabasalts of the mid-Archean Barberton greenstone belt, South Africa, have been argued to contain the earliest evidence for microbial subseafloor life. This includes candidate trace fossils in the form of titanite microtextures, and sulfur isotopic signatures of pyrite preserved in metabasaltic glass of the c. 3.472 Ga Hooggenoeg Formation. It has been contended that similar microtextures in altered martian basalts may represent potential extraterrestrial biosignatures of microbe-fluid-rock interaction. But despite numerous studies describing these putative early traces of life, a detailed metamorphic characterization of the microtextures and their host alteration conditions in the ancient pillow lava metabasites is lacking. Here, we present a new nondestructive technique with which to study the in situ metamorphic alteration conditions associated with potential biosignatures in mafic-ultramafic rocks of the Hooggenoeg Formation. Our approach combines quantitative microscale compositional mapping by electron microprobe with inverse thermodynamic modeling to derive low-temperature chlorite crystallization conditions. We found that the titanite microtextures formed under subgreenschist to greenschist facies conditions. Two chlorite temperature groups were identified in the maps surrounding the titanite microtextures and record peak metamorphic conditions at 315 ± 40°C (XFe3+(chlorite) = 25-34%) and lower-temperature chlorite veins/microdomains at T = 210 ± 40°C (lower XFe3+(chlorite) = 40-45%). These results provide the first metamorphic constraints in textural context on the Barberton titanite microtextures and thereby improve our understanding of the local preservation conditions of these potential biosignatures. We suggest that this approach may prove to be an important tool in future

  15. Pb-isotopic systematics of lunar highland rocks (>3.9 Ga): Constraints on early lunar evolution

    Premo, W.R.; Tatsumoto, M.; Misawa, K.; Nakamuka, N.; Kita, N.I.

    1999-01-01

    The present lead (Pb)-isotopic database of over 200 analyses from nearly 90 samples of non-mare basalt, lunar highland rocks (>3.9 Ga) delineate at least three isotopically distinct signatures that in some combination can be interpreted to characterize the systematics of the entire database. Two are fairly new sets of lunar data and are typical of Pb data from other solar-system objects, describing nearly linear arrays slightly above the 'geochron' values, with 207Pb/206Pb values 500). Although the age and origin of this exotic Pb is not well constrained, it is interpreted to be related to the entrapment of incompatible-element-rich (U, Th) melts within the lunar upper mantle and crust between 4.36 and 4.46 Ga (urKREEP residuum?). The latest discovered Pb signature is found only in lunar meteorites and is characterized by relatively low source ?? values between 10 and 50 at 3.9 Ga. The fact that most lunar crustal rocks (>3.9 Ga) exhibit high 207Pb/206Pb values requires that they were derived from, mixed with, or contaminated by Pb produced from early-formed, high-?? sources. The ubiquity of these U-Pb characteristics in the sample collection is probably an artifact of Apollo and Luna sampling sites, all located on the near side of the Moon, which was deeply excavated during the basin-forming event(s). However, the newest Pb-isotopic data support the idea that the Moon originally had a ?? value of ~8 to 35, slightly elevated from Earth values, and that progressive U-Pb fractionations occurred within the Moon during later stages of differentiation between 4.36 and 4.46 Ga.

  16. Eruptive style and construction of shallow marine mafic tuff cones in the Narakay Volcanic Complex (Proterozoic, Hornby Bay Group, Northwest Territories, Canada)

    NASA Astrophysics Data System (ADS)

    Ross, Gerald M.

    1986-03-01

    The Early Proterozoic (1663 Ma) Narakay Volcanic Complex, exposed in Great Bear Lake (Northwest Territories, Canada), is a bimodal suite of basalt and rhyolite erupted in a continental setting and consisting largely of pyroclastic rocks interlayered with shallow marine sedimentary rocks of the Hornby Bay Group. Mafic pyroclastic rocks consist of lapilli tuff, tuff, tuff breccia and agglomerate that represent the remnants of small subaerial tuff cones (0.5 to 2 km in diameter) that in most cases have subsided into the volcanic conduit. Stratification styles, sedimentary structures and grain morphologies in pyroclastic rocks reflect variations in the water:magma ratio during eruptions and have been used to help elucidate eruptive mechanisms and reconstruct volcanic edifices. Basaltic pyroclasts are commonly bounded by fracture surfaces and are morphologically similar to modern pyroclasts produced by thermal quench fragmentation or steam-blast disruption of magma. Most fragments have low vesicularity and scoria is only locally abundant which indicates that eruptive energy was supplied mostly by water—melt interaction rather than exsolution of magmatic gases. Cored bombs and lapilli, fusiform bombs, and pyroclasts similar in texture to those of Strombolian cinder and agglutinate spatter, are uncommon but are stratigraphically widespread and imply the occurrence of Strombolian eruptions, presumably when water access to the vent was impeded. Massive bedding is typical of the tuffs and, in addition to the poorly sorted ash-rich nature of the tuffs, implies deposition from water- and/or steam-rich hydrovolcanic eruption clouds and cypressoid jets by airfall and dense pyroclastic flows. Uncommon well-stratified and sorted ash and lapilli tuff record airfall and pyroclastic flow(?) deposition from eruption clouds rich in magmatic gases. Base surge deposits are uncommon and occur only in the subaerial portion of a sequence of tuffs inferred to record the progradation of a

  17. Proterozoic structure, cambrian rifting, and younger faulting as revealed by a regional seismic reflection network in the Southern Illinois Basin

    Potter, C.J.; Drahovzal, James A.; Sargent, M.L.; McBride, J.H.

    1997-01-01

    Four high-quality seismic reflection profiles through the southern Illinois Basin, totaling 245 km in length, provide an excellent regional subsurface stratigraphic and structural framework for evaluation of seismic risk, hydrocarbon occurrence, and other regional geologic studies. These data provide extensive subsurface information on the geometry of the intersection of the Cambrian Reelfoot and Rough Creek rifts, on extensive Proterozoic reflection sequences, and on structures (including the Fluorspar Area Fault Complex and Hicks Dome) that underlie a transitional area between the well-defined New Madrid seismic zone (to the southwest) and a more diffuse area of seismicity in the southern Illinois Basin. Our principal interpretations from these data are listed here in order of geologic age, from oldest to youngest: 1. Prominent Proterozoic layering, possibly equivalent to Proterozoic (???1 Ga) Middle Run Formation clastic strata and underlying (1.3-1.5 Ga) volcanic rocks of the East Continent rift basin, has been strongly deformed, probably as part of the Grenville foreland fold and thrust belt. 2. A well-defined angular unconformity is seen in many places between Proterozoic and Cambrian strata; a post-Grenville Proterozoic sequence is also apparent locally, directly beneath the base of the Cambrian. 3. We infer a major reversal in Cambrian rift polarity (accommodation zone) in the Rough Creek Graben in western Kentucky. 4. Seismic facies analysis suggests the presence of basin-floor fan complexes at and near the base of the Cambrian interval and within parts of a Proterozoic post-Grenville sequence in several parts of the Rough Creek Graben. 5. There is an abrupt pinchout of the Mount Simon Sandstone against crystalline basement beneath the Dale Dome (near the Texaco no. 1 Cuppy well, Hamilton County) in southeastern Illinois, and a more gradual Mount Simon pinchout to the southeast. 6. Where crossed by the seismic reflection line in southeast Illinois, some

  18. Root zone of the Late Proterozoic Salma Caldera, northeastern Arabian Shield, Kingdom of Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Kellogg, Karl S.

    1985-11-01

    The eroded root of the late Proterozoic Salma caldera crops out in a striking, roughly elliptical feature, about 27 km long and 22 km wide, near the northeastern edge of the Arabian Shield. The caldera is genetically part of an elongate alkalic granitic massif (Jabal Salma) that extends 35 km from the caldera to the southwest. Comenditic ash flow tuff and lava(?) of the caldera fill, probably more than 1 km thick, are the oldest recognized rocks of the caldera complex. These rocks were erupted during caldera collapse associated with the rapid evacuation of the upper, mildly peralkalic part of a zoned magma reservoir. Within the caldera fill, a massive, lithic-rich intracaldera rhyolite, probably a lava in excess of 1 km thick, is overlain by a layered ash flow sequence. Numerous megabreccia blocks, probably derived from the caldera wall, occur in the massive rhyolite. Open folds in the layered volcanic rocks may be due to high-temperature slumping of the rocks toward the center of the caldera following collapse. Later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill. After caldera collapse, metaluminous to peraluminous magma rose beneath the caldera at approximately 580 Ma and solidified as biotite alkali granite, rim syenogranite, and late, high-level granophyre. Rare earth element abundances indicate that the layered rhyolite tuff, peralkalic granite, and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite. The granophyre intruded the caldera fill as a dome-shaped body composed of numerous sheetlike masses. Granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet. Maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera, where the caldera fill volcanic rocks have been removed by erosion. No apparent structural

  19. Geology and geochronology of granitoid and metamorphic rocks of late Archean age in northwestern Wisconsin

    Sims, P.K.; Peterman, Z.E.; Zartman, R.E.; Benedict, F.C.

    1985-01-01

    Granitoid rocks of the Puritan Quartz Monzonite and associated biotite gneiss and amphibolite in northwestern Wisconsin compose the southwestern part of the Puritan batholith of Late Archean age. They differ from rocks in the Michigan segment of the batholith in having been deformed by brittle-ductile deformation and partly recrystallized during shearing accompanying development of the midcontinent rift system of Keweenawan (Middle Proterozoic) age. Granitoid rocks ranging in composition from granite to tonalite are dominant in the Wisconsin part of the batholith. To the north of the Mineral Lake fault zone, they are massive to weakly foliated and dominantly of granite composition, whereas south of the fault zone they are more strongly foliated and mainly of tonalite composition. Massive granite, leucogranite, and granite pegmatite cut the dominant granitoid rocks. Intercalated with the granitoid rocks in small to large conformable bodies are biotite gneiss, amphibolite, and local tonalite gneiss. Metagabbro dikes of probable Early Proterozoic age as much as 15 m thick cut the Archean rocks. Rubidium-strontium whole-rock data indicate a Late Archean age for the granitoids and gneisses, but data points are scattered and do not define a single isochron. Zircon from two samples of tonalitic gneiss for uranium-thorium-Iead dating define a single chord on a concordia diagram, establishing an age of 2,735?16 m.y. The lower intercept age of 1,052?70 m.y. is in close agreement with rubidium-strontium and potassium-argon biotite ages from the gneisses. Two episodes of deformation and metamorphism are recorded in the Archean rocks. Deformation during the Late Archean produced a steep west-northwest-oriented foliation and gently plunging fold axes and was accompanied by low amphibolite-facies metamorphism of the bedded rocks. A younger deformation resulting from largely brittle fracture was accompanied by retrogressive metamorphism; this deformation is most evident adjacent

  20. Refined Proterozoic evolution of the Gawler Craton, South Australia, through U-Pb zircon geochronology

    Fanning, C.M.; Flint, R.B.; Parker, A.J.; Ludwig, K. R.; Blissett, A.H.

    1988-01-01

    Through the application of both conventional U-Pb zircon analyses and small-sample U-Pb isotopic analyses, the nature and timing of tectonic events leading to the formation of the Gawler Craton have been defined more precisely. Constraints on deposition of Early Proterozoic iron formation-bearing sediments have been narrowed down to the period 1960-1847 Ma. Deformed acid volcanics, including the economically important Moonta Porphyry, have zircon ages of ??? 1790 and 1740 Ma. The voluminous acid Gawler Range Volcanics and correlatives to the east were erupted over a short interval at 1592 ?? 2 Ma, and were intruded by anorogenic granites at ??? 1575 Ma. Small-sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses, generally yielding more-concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight-line discordia. ?? 1988.

  1. Preliminary source rock evaluation and hydrocarbon generation potential of the early Cretaceous subsurface shales from Shabwah sub-basin in the Sabatayn Basin, Western Yemen

    NASA Astrophysics Data System (ADS)

    Al-Matary, Adel M.; Hakimi, Mohammed Hail; Al Sofi, Sadam; Al-Nehmi, Yousif A.; Al-haj, Mohammed Ail; Al-Hmdani, Yousif A.; Al-Sarhi, Ahmed A.

    2018-06-01

    A conventional organic geochemical study has been performed on the shale samples collected from the early Cretaceous Saar Formation from the Shabwah oilfields in the Sabatayn Basin, Western Yemen. The results of this study were used to preliminary evaluate the potential source-rock of the shales in the Saar Formation. Organic matter richness, type, and petroleum generation potential of the analysed shales were assessed. Total organic carbon content and Rock- Eval pyrolysis results indicate that the shale intervals within the early Cretaceous Saar Formation have a wide variation in source rock generative potential and quality. The analysed shale samples have TOC content in the range of 0.50 and 5.12 wt% and generally can be considered as fair to good source rocks. The geochemical results of this study also indicate that the analysed shales in the Saar Formation are both oil- and gas-prone source rocks, containing Type II kerogen and mixed Types II-III gradient to Type III kerogen. This is consistent with Hydrogen Index (HI) values between 66 and 552 mg HC/g TOC. The temperature-sensitive parameters such as vitrinite reflectance (%VRo), Rock-Eval pyrolysis Tmax and PI reveal that the analysed shale samples are generally immature to early-mature for oil-window. Therefore, the organic matter has not been altered by thermal maturity thus petroleum has not yet generated. Therefore, exploration strategies should focus on the known deeper location of the Saar Formation in the Shabwah-sub-basin for predicting the kitchen area.

  2. A deposit model for magmatic iron-titanium-oxide deposits related to Proterozoic massif anorthosite plutonic suites

    Woodruff, Laurel G.; Nicholson, Suzanne W.; Fey, David L.

    2013-01-01

    This descriptive model for magmatic iron-titanium-oxide (Fe-Ti-oxide) deposits hosted by Proterozoic age massif-type anorthosite and related rock types presents their geological, mineralogical, geochemical, and geoenvironmental attributes. Although these Proterozoic rocks are found worldwide, the majority of known deposits are found within exposed rocks of the Grenville Province, stretching from southwestern United States through eastern Canada; its extension into Norway is termed the Rogaland Anorthosite Province. This type of Fe-Ti-oxide deposit dominated by ilmenite rarely contains more than 300 million tons of ore, with between 10- to 45-percent titanium dioxide (TiO2), 32- to 45-percent iron oxide (FeO), and less than 0.2-percent vanadium (V). The origin of these typically discordant ore deposits remains as enigmatic as the magmatic evolution of their host rocks. The deposits clearly have a magmatic origin, hosted by an age-constrained unique suite of rocks that likely are the consequence of a particular combination of tectonic circumstances, rather than any a priori temporal control. Principal ore minerals are ilmenite and hemo-ilmenite (ilmenite with extensive hematite exsolution lamellae); occurrences of titanomagnetite, magnetite, and apatite that are related to this deposit type are currently of less economic importance. Ore-mineral paragenesis is somewhat obscured by complicated solid solution and oxidation behavior within the Fe-Ti-oxide system. Anorthosite suites hosting these deposits require an extensive history of voluminous plagioclase crystallization to develop plagioclase-melt diapirs with entrained Fe-Ti-rich melt rising from the base of the lithosphere to mid- and upper-crustal levels. Timing and style of oxide mineralization are related to magmatic and dynamic evolution of these diapiric systems and to development and movement of oxide cumulates and related melts. Active mines have developed large open pits with extensive waste-rock piles, but

  3. Oxygen isotope studies of early Precambrian granitic rocks from the Giants Range batholith, northeastern Minnesota, U.S.A.

    Viswanathan, S.

    1974-01-01

    Oxygen isotope studies of granitic rocks from the 2.7 b.y.-old composite Giants Range batholith show that: (1) ??(O18)quartz values of 9 to 10 permil characterize relatively uncontaminated Lower Precambrian, magmatic granodiorites and granites; (2) granitic rocks thought to have formed by static granitization have ??(O18)quartz values that are 1 to 2 permil higher than magmatic granitic rocks; (3) satellite leucogranite bodies have values nearly identical to those of the main intrusive phases even where they transect O18-rich metasedimentary wall rocks; (4) oxygen isotopic interaction between the granitic melts and their O18-rich wall rocks was minimal; and (5) O18/O18 ratios of quartz grains in a metasomatic granite are largely inherited from the precursor rock, but during the progression - sedimentary parent ??? partially granitized parent ??? metasomatic granite ??? there is gradual decrease in ??(O18)quartz by 1 to 2 permil. ?? 1974.

  4. Reconstruction of an early Paleozoic continental margin based on the nature of protoliths in the Nome Complex, Seward Peninsula, Alaska

    Till, Alison B.; Dumoulin, Julie A.; Ayuso, Robert A.; Aleinikoff, John N.; Amato, Jeffrey M.; Slack, John F.; Shanks, W.C. Pat

    2014-01-01

    The Nome Complex is a large metamorphic unit that sits along the southern boundary of the Arctic Alaska–Chukotka terrane, the largest of several micro continental fragments of uncertain origin located between the Siberian and Laurentian cratons. The Arctic Alaska–Chukotka terrane moved into its present position during the Mesozoic; its Mesozoic and older movements are central to reconstruction of Arctic tectonic history. Accurate representation of the Arctic Alaska–Chukotka terrane in reconstructions of Late Proterozoic and early Paleozoic paleogeography is hampered by the paucity of information available. Most of the Late Proterozoic to Paleozoic rocks in the Alaska–Chukotka terrane were penetratively deformed and recrystallized during the Mesozoic deformational events; primary features and relationships have been obliterated, and age control is sparse. We use a variety of geochemical, geochronologic, paleontologic, and geologic tools to read through penetrative deformation and reconstruct the protolith sequence of part of the Arctic Alaska–Chukotka terrane, the Nome Complex. We confirm that the protoliths of the Nome Complex were part of the same Late Proterozoic to Devonian continental margin as weakly deformed rocks in the southern and central part of the terrane, the Brooks Range. We show that the protoliths of the Nome Complex represent a carbonate platform (and related rocks) that underwent incipient rifting, probably during the Ordovician, and that the carbonate platform was overrun by an influx of siliciclastic detritus during the Devonian. During early phases of the transition to siliciclastic deposition, restricted basins formed that were the site of sedimentary exhalative base-metal sulfide deposition. Finally, we propose that most of the basement on which the largely Paleozoic sedimentary protolith was deposited was subducted during the Mesozoic.

  5. Synchronous partial melting, deformation, and magmatism: evidence from in an exhumed Proterozoic orogen

    NASA Astrophysics Data System (ADS)

    Levine, J. S. F.; Mosher, S.

    2017-12-01

    Older orogenic belts that now expose the middle and lower crust record interaction between partial melting, magmatism, and deformation. A field- and microstructural-based case study from the Wet Mountains of central Colorado, an exhumed section of Proterozoic rock, shows structures associated with anatexis and magmatism, from the grain- to the kilometer-scale, that indicate the interconnection between deformation, partial melting, and magmatism, and allow reconstructions of the processes occurring in hot active orogens. Metamorphic grade, along with the degree of deformation, partial melting, and magmatism increase from northwest to southeast. Deformation synchronous with this high-grade metamorphic event is localized into areas with greater quantities of former melt, and preferential melting occurs within high-strain locations. In the less deformed northwest, partial melting occurs dominantly via muscovite-dehydration melting, with a low abundance of partial melting, and an absence of granitic magmatism. The central Wet Mountains are characterized by biotite dehydration melting, abundant former melt and foliation-parallel inferred melt channels along grain boundaries, and the presence of a nearby granitic pluton. Rocks in the southern portion of the Wet Mountains are characterized by partial melting via both biotite dehydration and granitic wet melting, with widespread partial melting as evidenced by well-preserved former melt microstructures and evidence for back reaction between melt and the host rocks. The southern Wet Mountains has more intense deformation and widespread plutonism than other locations and two generations of dikes and sills. Recognition of textures and fabrics associated with partial melting in older orogens is paramount for interpreting the complex interplay of processes occurring in the cores of orogenic systems.

  6. Detrital and early chemical remanent magnetization in redbeds and their rock magnetic signature: Zicapa Formation, southern Mexico

    NASA Astrophysics Data System (ADS)

    Sierra-Rojas, Maria Isabel; Molina-Garza, Roberto Stanley

    2018-06-01

    Poles from continental redbeds are a large fraction of the world's palaeomagnetic database. Nonetheless, the time of acquisition and origin of the remanent magnetization of redbeds has been long debated. We report palaeomagnetic data, rock magnetic data and microscope observations for Lower Cretaceous redbeds in southern Mexico. These data allow us to discriminate between the hysteresis properties of remanent magnetizations of detrital and chemical origin, and to establish the early origin of a chemical remanence. Red sandstones of the Zicapa Formation contain a multicomponent remanence revealed by thermal demagnetization, and consisting of three stable components with partially overlapping laboratory unblocking temperatures of <250 °C, ˜300 to ˜500 °C and >600 °C, (low, intermediate and high temperature, respectively). They are interpreted as a viscous remanence residing in detrital magnetite, a chemical remanence residing in authigenic hematite and a depositional remanence residing in detrital hematite, respectively. The low-temperature component is nearly parallel to the recent dipole field. The tilt-corrected overall site means of the intermediate (chemical) and high temperature (depositional) components are indistinguishable (Dec = 282.0°, Inc = 12.4°, k = 13.33, α95 = 10.1°, N = 17, for the intermediate temperature; and Dec = 272.5°, Inc = 16.5°, k = 14.04, α95 = 11, N = 14, for the high temperature). Elongation/inclination analysis suggests that depositional and chemical components require applying an f = factor of approximately 0.4. Both of these components define a magnetic polarity zonation, but the polarity of the chemical and detrital components may or may not be the same. The chemical remanence coincides, more often than not, with the polarity of the depositional remanence of the overlying (younger) strata, suggesting a delay in remanence acquisition of tens to a few hundred ka for the chemical component. Pigmentary and detrital haematite

  7. Paleoproterozoic mojaveprovince in northwestern Mexico? Isotopic and U-Pb zircon geochronologic studies of precambrian and Cambrian crystalline and sedimentary rocks, Caborca, Sonora

    Lang, Farmer G.; Bowring, S.A.; Matzel, J.; Maldonado, G.E.; Fedo, C.; Wooden, J.

    2005-01-01

    Whole-rock Nd isotopic data and U-Pb zircon geochronology from Precambrian crystalline rocks in the Caborca area, northern Sonora, reveal that these rocks are most likely a segment of the Paleoproterozoic Mojave province. Supporting this conclusion are the observations that paragneiss from the ??? 1.75 Ga Bamori Complex has a 2.4 Ga Nd model age and contains detrital zircons ranging in age from Paleo- proterozoic (1.75 Ga) to Archean (3.2 Ga). Paragneisses with similar age and isotopic characteristics occur in the Mojave province in southern California. In addition, "A-type" granite exposed at the southern end of Cerro Rajon has ca 2.0 Ga Nd model age and a U-Pb zircon age of 1.71 Ga, which are similar to those of Paleoproterozoic granites in the Mojave province. Unlike the U.S. Mojave province, the Caborcan crust contains ca. 1.1 Ga granite (Aibo Granite), which our new Nd isotopic data suggest is largely the product of anatexis of the local Precambrian basement. Detrital zircons from Neoproterozoic to early Cambrian miogeoclinal arenites at Caborca show dominant populations ca. 1.7 Ga, ca. 1.4 Ga, and ca. 1.1 Ga, with subordinate Early Cambrian and Archean zircons. These zircons were likely derived predominately from North American crust to the east and northeast, and not from the underlying Caborcan basement. The general age and isotopic similarities between Mojave province basement and overlying miogeoclinal sedimentary rocks in Sonora and southern California is necessary, but not sufficient, proof of the hypothesis that Sonoran crust is allochthonous and was transported to its current position during the Mesozoic along the proposed Mojave-Sonora megashear. One viable alternative model is that the Caborcan Precambrian crust is an isolated, autochthonous segment of Mojave province crust that shares a similar, but not identical, Proterozoic geological history with Mojave province crust found in the southwest United States ?? 2005 Geological Society of America.

  8. Origin of sulfide and phosphate deposits in Upper Proterozoic carbonate strata, Irece basin, Bahia, Brazil

    SciT

    Kyle, J.R.; Misi, A.

    1991-03-01

    Carbonate strata of the Una Group represent late Proterozoic platform sedimentation in the Irece basin of north-central Brazil. Stratabound sulfide- and phosphate-rich units occur within a 50-m thick tidal flat sequence of dolomitic limestone and cherty dolomite. Three types of primary phosphate concentrations are present: columnar stromatolitic, laminar stromatolitic, and intraclastic. Resedimented phosphate clasts and phosphatic units interbedded with non phosphatic dolomites suggest early diagenetic replacement of algal carbonate units. Local stratabound Zn-Pb-Ag sulfide concentrations at the Tres Irmas prospect occur within silty dolomite with shallow water sedimentary structures and local disturbed laminae, synsedimentary faults, and breccias. Sulfide minerals includemore » pyrite, sphalerite, galena, marcasite, jordanite, tetrahedrite, and covellite. Pyrite crystal aggregates commonly show bladed forms. Nodular aggregates of length-slow quartz are locally associated with sulfides. Sulfur isotope analyses indicate relatively uniform heavy {delta}{sup 34}S values. Barite shows a {delta}{sup 34}S range from +25.2 to +29.6{per thousand}, CDT. Pyrite and sphalerite representative of a variety of textural types have a {delta}{sup 34}S range of +20.2 to +22.6{per thousand}. Late Proterozoic evaporite sulfates show a wide range of {delta}{sup 34} S values from about +10 to +28{per thousand}. Thus, the {delta}{sup 34}S values for Irece barite could reflect original seawater sulfate values. However, the relatively heavy {delta}{sup 34}S values of the associated sulfides suggests that the original seawater sulfate was modified by bacterial sulfate reduction processes in shallow sea floor sediments. Textural and {delta}{sup 34}S evidence suggests that a later stage of metallic mineralization scavenged sulfur from preexisting sulfides or from direct reduction of evaporitic sulfate minerals.« less

  9. Sm-Nd and Rb-Sr isotopic systematics of the Pea Ridge Fe-P deposit and related rocks, southeast Missouri

    SciT

    Marikos, M.A.; Barton, M.D.

    1993-03-01

    Pea ridge is a discordant Middle Proterozoic Fe-P deposit hosted in rhyolite tuffs and flows of the 1.4--1.5 Ga St. Francois terrane. Host rocks and the deposit are cut by basalt and aplite/pegmatite dikes. The deposit overlies a blind pluton which is partially surrounded by a trachytic ring complex. In the deposit, which is mined for Fe, early Qtz+Amph+Mag+Ap rock is cut by Mag+Ap+Qtz rock. Subsequently, portions of the deposit and host rocks were brecciated, oxidized and silicified to produce a complex suite of rocks enriched in Hem+Qtz+Ksp+Mu. Late breccia pipes/dikes cut the complex and were mineralized with Bar+Ksp+Flu+Chl+Cc+REE-phosphates. Sm/Ndmore » and Rb/Sr isotopic systematics have been studied to: (1) constrain source(s) of igneous rocks and deposit components, (2) refine ages of magmatism, mineralization, and later hydrothermal activity, (3) begin regional comparison of isotopic systematics in SE Missouri Fe deposits, and (4) complement ongoing Missouri DGLS/USGS studies. Fourteen combined Sm-Nd and Rb-Sr analyses were done on materials including two host rhyolites, two nearby trachytes, two gneiss samples representing plausible basement, two intramineral dikes, and six samples of mineralization.« less

  10. Middle Proterozoic uplift events in the Dunbar dome of northeastern Wisconsin, USA

    Peterman, Z.E.; Sims, P.K.; Zartman, R.E.; Schulz, K.J.

    1985-01-01

    Isotopic ages of granitic and metamorphic rocks exposed in the Dunbar structural dome of northeastern Wisconsin identify a protracted series of tectonic and "hydrothermal" events that culminated in major regional uplift during Middle Proterozoic (Keweenawan; ca 1,100 Ma) continental rifting and volcanism. The major rock-forming events and the structural development of the dome occurred during the interval 1,862+/-4 Ma to 1,836+/-6 Ma. Whole-rock Rb-Sr ages are partly reset in response to a widely recognized but cryptic event in Wisconsin and Michigan at about 1,630 Ma. The scale and systematic character of the whole-rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low-grade metamorphic rocks. The regional nature of the 1,630-Ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south. Rb-Sr biotite ages for the Dunbar dome (this study), the southern complex of the Marquette district (Van Schmus and Woolsey 1975) and the Felch trough area (Aldrich and others 1965) provide a remarkably coherent pattern that reflects multiple episodes of differential uplift. Younger events superimposed on a regional 1,630-Ma imprint are recorded at 1,330 Ma and 1,140 Ma. The 1,330 Ma disturbance could reflect stabilization following intrusion of the Wolf River batholith at 1,485 Ma. The 1,140-Ma uplift event occurred during Keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault-bounded blocks with possible subcrustal influence. The remarkably small variance in the 1,140-Ma biotite age peak argues for rapid uplift and cooling, and hence rapid erosion. Detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion. ?? 1985 Springer-Verlag.

  11. Trace element differences between Archean, Proterozoic and Phanerozoic crustal components: Implications for crustal growth processes

    NASA Technical Reports Server (NTRS)

    Tarney, J.; Wyborn, L. E. A.; Sheraton, J. W.; Wyborn, D.

    1988-01-01

    Critical to models for continental crust growth and recycling are the processes through which crustal growth takes place. In particular, it is important to know whether these processes have changed fundamentally with time in response to the earth's thermal evolution, and whether the crustal compositions generated are compatible with crustal remobilization, crustal recycling, or represent primary additions. There are some significant and consistent differences in the major and trace element compositions of crustal components with time which have important implications for crustal growth processes. These will be illustrated with reference to Archean rocks from a number of shield areas, Proterozoic granitoids from Australia and elsewhere, Palaeozoic granitoids from Australia and Scotland, and Mesozoic - recent granitoids from present continental margin belts. Surprisingly some rather simple and consistent patterns energy using this technique. There are then significant differences in compositions of granitoid crustal additions throughout geological time, with a particular type of granitoid apparently dominating a particular time period. This implies that the tectonic processes giving rise to granite generation have changed in response to the earth's thermal evolution.

  12. Possible polyphase metamorphic evolution of high grade metabasic rocks from the Songshugou ophiolite, Qinling orogen, China

    NASA Astrophysics Data System (ADS)

    Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng; Chen, Danling

    2014-05-01

    The Proterozoic Songshugou ophiolite consists of a series of ultrabasic and tholeitic metabasic rocks. They were emplaced as a lense shaped body into the southern margin of the Qinling Group. Isotope composition and trace element geochemistry display an E-MORB and T-MORB signature for the mafic rocks (Dong et al., 2008). Within the ophiolite sequence some rudimental fresh peridotites (dunites and harzburgites) within serpentines display low CaO (<0.39 wt.%) and Al2O3 (<0.51 wt.%) as well as high MgO (41-48 wt.%) contents, which can be classified as depleted non-fertile mantle rocks. The metabasic rocks comprise the mineral assemblage garnet, amphibole, symplectitic pyroxenes, ilmenite, apatite, ±zoisite, ±sphene and show a strong retrograde metamorphic overprint. Garnet typically contains many inclusions within the core but are nearly inclusion free at the rim. The cores have sometimes snowball textures indicating initially syndeformative growth. Albite and prehnite were found in central parts of garnet. In the outer portions, pargasitic amphibole, rutile and a bluish amphibole, probably glaukophane were found. Garnet zoning pattern clearly show a discontinous growth seen in an sudden increase in grossular and decrease in almandine components. The symplectitic pyroxenes are of diopsidic composition which enclose typically prehnite and not albite, as common in retrograde eclogitic rocks. Different stages of garnet breakdown to plagioclase and amphibole, from thin plagioclase rims surrounding the garnets to plagioclase rich pseudomorphs, can be observed in different samples. Based on symplectitic pyroxenes a high pressure metamorphic event can be concluded (Zhang, 1999). The garnet breakdown to plagioclase and the symplectites clearly indicate a rapid exhumation phase. The age of the metamorphic event is probably related to the closure of the Shangdan ocean during the early Paleozoic. It is unclear if the garnet rims grew during a later stage of the metamorphic

  13. Geochemistry, geochronology, and tectonic setting of Early Cretaceous volcanic rocks in the northern segment of the Tan-Lu Fault region, northeast China

    NASA Astrophysics Data System (ADS)

    Ling, Yi-Yun; Zhang, Jin-Jiang; Liu, Kai; Ge, Mao-Hui; Wang, Meng; Wang, Jia-Min

    2017-08-01

    We present new geochemical and geochronological data for volcanic and related rocks in the regions of the Jia-Yi and Dun-Mi faults, in order to constrain the late Mesozoic tectonic evolution of the northern segment of the Tan-Lu Fault. Zircon U-Pb dating shows that rhyolite and intermediate-mafic rocks along the southern part of the Jia-Yi Fault formed at 124 and 113 Ma, respectively, whereas the volcanic rocks along the northern parts of the Jia-Yi and Dun-Mi faults formed at 100 Ma. The rhyolite has an A-type granitoid affinity, with high alkalis, low MgO, Ti, and P contents, high rare earth element (REE) contents and Ga/Al ratios, enrichments in large-ion lithophile (LILEs; e.g., Rb, Th, and U) and high-field-strength element (HFSEs; e.g., Nb, Ta, Zr, and Y), and marked negative Eu anomalies. These features indicate that the rhyolites were derived from partial melting of crustal material in an extensional environment. The basaltic rocks are enriched in light REEs and LILEs (e.g., Rb, K, Th, and U), and depleted in heavy REEs, HFSEs (e.g., Nb, Ta, Ti, and P), and Sr. These geochemical characteristics indicate that these rocks are calc-alkaline basalts that formed in an intraplate extensional tectonic setting. The dacite is a medium- to high-K, calc-alkaline, I-type granite that was derived from a mixed source involving both crustal and mantle components in a magmatic arc. Therefore, the volcanic rocks along the Jia-Yi and Dun-Mi faults were formed in an extensional regime at 124-100 Ma (Early Cretaceous), and these faults were extensional strike-slip faults at this time.

  14. Early Precambrian mantle derived rocks in the southern Prince Charles Mountains, East Antarctica: age and isotopic constraints

    Mikhalsky, E.V.; Henjes-Kunst, F.; Roland, N.W.

    2007-01-01

    Mafic and ultramafic rocks occurring as lenses, boudins, and tectonic slabs within metamorphic units in the southern Mawson Escarpment display mantle characteristics of either a highly enriched, or highly depleted nature. Fractionation of these mantle rocks from their sources may be as old as Eoarchaean (ca 3850 Ma) while their tectonic emplacement probably occurred prior to 2550 Ma (U-Pb SHRIMP data). These results provide for the first time evidence for Archaean suturing within East Antarctica. Similar upper mantle sources are likely present in the northern Mawson Escarpment. A younger age limit of these rocks is 2200 Ma, as indicated by presumably metamorphic zircon ages while their magmatic age may be constrained by single zircon dates at 2450-2250 Ma. The area of the northern Mawson Escarpment is most likely of ensimatic origin and includes mafic rocks which were derived from distinct mantle source(s) during Palaeoproterozoic time.

  15. Post-Panafrican late Proterozoic basins in the Central Anti-Atlas (Morocco): their influence on the Variscan contractional structures.

    NASA Astrophysics Data System (ADS)

    Guimerà, Joan; Arboleya, María. Luisa

    2010-05-01

    Located South of the High Atlas, in Morocco, The Anti-Atlas is a 700 km-long chain trending NE-SW. In the Central Anti-Atlas region, between Warzazat and Taznakht, the Proterozoic Pan-African basement (X1 to X2-3) crops out in isolated areas (boutonnières), where it is overlaid by late to post Pan-African Upper Proterozoic and Palaeozoic rocks. Late to post Pan-African Upper Proterozoic rocks (X3) have been classically divided into three units (X3i, X3m and X3s) which include volcanic rocks — mainly rhyolites— and continental siliciclastic rocks, the older units intruded by late granites (Choubert, 1952 and Choubert et al., 1970). Rocks belonging to the upper unit of post Pan-African Upper Proterozoic rocks (X3s) were deposited in basins bounded by faults with a dominant dip-slip normal motion; as a result, this unit have a variable thickness, being locally absent in the uplifted blocks. Uppermost Proterozoic (Adoudounian) and Palaeozoic rocks deposited unconformable on the older rocks in the Anti-Atlas. The Central Anti-Atlas was slightly deformed during the Variscan orogeny by folds and high-angle thrusts. Two areas are selected to study the post Pan-African to Variscan evolution of the area: the Tiwiyyine basin and the Anti-Atlas Major Fault. Tiwiyyine basin This basin is delimited by kilometric-scale normal faults. Three of them can be observed in the field: two striking NE-SW (NW and SE boundaries) and one striking NW-SE (SW boundary), while the NE boundary is covered by Cenozoic rocks. The basin fill reaches 725 m and has been divided into three units: 1. X3s1: Coarse conglomerates with basal breccias. 2. X3s2: Laminated dolomites at the base, red pelites and conglomerates. 3. X3s3: Conglomerates with interbedded andesites. Unit X3s2 passes laterally to the SW to unit X3s1. The thickness of the basin fill diminishes to the SE. This is specially visible at the basal X3s1 unit. At both sides of the two NE-SW-striking faults, only the upper X3s3 unit is

  16. Igneous Rocks

    NASA Astrophysics Data System (ADS)

    Doe, Bruce R.

    “Igneous Rocks was written for undergraduate geology majors who have had a year of college-level chemistry and a course in mineralogy … and for beginning graduate students. Geologists working in industry, government, or academia should find this text useful as a guide to the technical literature up to 1981 and as an overview of topics with which they have not worked but which may have unanticipated pertinence to their own projects.” So starts the preface to this textbook.As one who works part time in research on igneous rocks, especially as they relate to mineral deposits, I have been looking for such a book with this avowed purpose in a field that has a choking richness of evolving terminology and a bewildering volume of interdisciplinary literature. In addition to the standard topics of igneous petrology, the book contains a chapter on the role of igneous activity in the genesis of mineral deposits, its value to geothermal energy, and the potential of igneous rocks as an environment for nuclear waste disposal. These topics are presented rather apologetically in the preface, but the author is to be applauded for including this chapter. The apology shows just how new these interests are to petrology. Recognition is finally coming that, for example, mineral deposits are not “sports of nature,” a view held even by many economic geologists as recently as the early 1960's; instead they are perfectly ordinary geochemical features formed by perfectly ordinary geologic processes. In fact, the mineral deposits and their attendant alteration zones probably have as much to tell us about igneous rocks as the igneous rocks have to tell us about mineral deposits.

  17. How Strong is the Case for Proterozoic Low-Latitude Glaciation?

    NASA Astrophysics Data System (ADS)

    Evans, D. A.

    2004-05-01

    The most recent global compilations of paleomagnetic depositional latitudes for Proterozoic glaciogenic formations indicate a dominant mode near the paleo-equator (Evans 2000 AJS; Evans 2003 Tectonophysics). This result would therefore support either the snowball Earth or the large-obliquity hypotheses for Precambrian ice ages, but would reject the uniformitarian comparison to polar-temperate-restricted Phanerozoic glaciogenic deposits. The most reliable low-latitude results come from the Australian Marinoan succession, but a recent summary of these units has suggested that a glaciogenic origin is not yet demonstrated (Eyles and Januszczak 2004 Earth-Sci Reviews). It becomes useful, then, to review the global evidence for Proterozoic low-latitude glaciation. Eyles and Januszczak (ibid.) identified 13 Neoproterozoic deposits with "demonstrated" glacial influence. Among these, poor age constraints and lack of paleomagnetic data prohibit estimation of depositional paleolatitudes for the Fiq, Sturtian, Vreeland, Taoudeni, East Greenland, Port Askaig, and Zhengmuguan units. Moderate paleolatitudes are reasonably well supported for the South China, Gaskiers, Smalfjord, and Moelv units. Among the three remaining units, the Rapitan Group can be assigned a near-equatorial paleolatitude indirectly through use of the Galeros and Franklin-Natkusiak paleomagnetic results, as long as the Rapitan age lies within 750-720 Ma as generally expected. The Moonlight Valley Formation in northern Australia may be assigned a tropical paleolatitude according to high-quality paleomagnetic results from compellingly correlated Marinoan strata in southern Australia. Those strata, including the famous Elatina Formation, have yielded a robust paleomagnetic signature that is commonly interpreted to imply frigid climate (manifest in part by frost-wedge polygons) at near-equatorial latitudes. Concerns that the Neoproterozoic geomagnetic field was either nonaxial or nondipolar are valid in principle

  18. Evidence for ancient atmospheric xenon in Archean rocks and implications for the early evolution of the atmosphere

    NASA Astrophysics Data System (ADS)

    Pujol, M.; Marty, B.; Burnard, P.; Hofmann, A.

    2012-12-01

    The initial atmospheric xenon isotopic composition has been much debated over the last 4 decades. A Non radiogenic Earth Atmospheric xenon (NEA-Xe) composition has been proposed to be the best estimate of the initial signature ([1]). NEA-Xe consists of modern atmospheric Xe without fission (131-136Xe) or radioactive decay (129Xe) products. However, the isotope composition of such non-radiogenic xenon is very different to that of potential cosmochemical precursors such as solar or meteoritic Xe, as it is mass-fractionated by up to 3-4 % per amu relative to the potential precursors, and it is also elementally depleted relative to other noble gases. Because the Xe isotopic composition of the Archean appears to be intermediate between that of these cosmochemical end-members and that of the modern atmosphere, we argued that isotopic fractionation of atmospheric xenon did not occur early in Earth's history by hydrodynamic escape, as postulated by all other models ([1], [2], [3]), but instead was a continuous, long term process that lasted during at least the Hadean and Archean eons. Taken at face value, the decrease of the Xe isotopic fractionation from 1.6-2.1 % amu-1 3.5 Ga ago ([4]) to 1 % amu-1 3.0 Ga ago (Ar-Ar age in fluid inclusions trapped in quartz from the same Dresser Formation, [5]) could reflect a secular variation of the atmospheric Xe signature. Nevertheless, up until now, all data showing an isotopic mass fractionation have been measured in rocks and fluids from the same formation (Dresser Formation, Western Australia, aged 3.5 Ga), and have yet to be confirmed in rocks from different locations. In order to better constrain xenon isotopic fractionation of the atmosphere through time, we decided to analyze barites from different ages, geological environments and metamorphism grade. We started this study with barite from the Fig Tree Formation (South Africa, aged 3.26 Ga). This barite was sampled in old mines so have negligible modern exposure time. It is

  19. U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

    NASA Astrophysics Data System (ADS)

    Otamendi, Juan E.; Ducea, Mihai N.; Cristofolini, Eber A.; Tibaldi, Alina M.; Camilletti, Giuliano C.; Bergantz, George W.

    2017-07-01

    The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U-Th-Pb and Lu-Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472-465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (-0.3 to -8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc-alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

  20. Proterozoic events recorded in quartzite cobbles at Jack Hills, Western Australia: New constraints on sedimentation and source of > 4 Ga zircons

    NASA Astrophysics Data System (ADS)

    Grange, Marion L.; Wilde, Simon A.; Nemchin, Alexander A.; Pidgeon, Robert T.

    2010-03-01

    Rare heavy mineral bands within quartzite cobbles were identified in two conglomerate units within the Jack Hills belt, Western Australia. Seven zircon-bearing cobbles were analysed from one location (site 152) and three from another (site 154), both approximately 1 km west of the site where zircons in excess of 4 Ga are abundant (W74 'discovery' site). Individual pebbles from the 152 site reveal three distinctive features, containing either zircons > 3.0 Ga in age, < 1.9 Ga in age or a range of ages from ˜ 1.2 to ˜ 3.6 Ga. Those from site 154 are more uniform, containing only zircons with ages between 3.1 and 3.9 Ga. Only one grain > 4 Ga was discovered from the entire suite of pebbles, in contrast to the well-studied W74 site. A single detrital zircon with an age of 1220 ± 42 Ma from location 152 is the youngest grain so far reported from sedimentary rocks at Jack Hills. It shows magmatic oscillatory zoning and thus implies at least two sedimentary cycles within the Proterozoic; requiring erosion of an igneous precursor, incorporation into a clastic sediment, induration and subsequent erosion and transport to be hosted in the conglomerate. The nearest source for rocks of this age is the Bangemall Supergroup in the Collier Basin, ˜ 100 km northeast in the Capricorn Orogen. This would imply tectonic interleaving of originally more extensive Bangemall rocks, possibly related to activity along the Cargarah Shear Zone that traverses the Jack Hills belt. The lack of > 4.1 Ga zircons in the pebbles is highly significant, suggesting the immediate source of ancient zircons was no longer present at the Earth's surface. This equates with a general lack of ancient crystals noted in rocks that contain Proterozoic zircons from previous studies and implies that such grains diminish in number as earlier sedimentary rocks were successively recycled.

  1. Laramide alteration of proterozoic diabase: A likely contributor of copper to porphyry systems in the dripping spring mountains area, Southeastern Arizona

    Force, E.R.

    1998-01-01

    Proterozoic diabase of the Dripping Spring range occurs as sills in the Proterozoic Apache Group and the Troy Quartzite and as intrusive sheets in basement rocks. The aggregate thickness of the diabase sills and intrusive sheets averages about 450 m in the part of the range showing little mid-Tertiary extension. Laramide alteration is of two types, dominated by chlorite and actinolite, respectively, and formed mostly from clinopyroxene. Actinolite-dominated assemblages are higher in Na and Ca. Hydrothermal biotite is common in the central areas of both alteration types. Laramide alteration forms two distribution patterns: a subequant pattern centered on Laramide intrusions and small porphyry deposits, characterized by actinolitic alteration, and a more extensive branching linear pattern that follows Laramide structures, centered on the larger Ray porphyry deposit, extending toward other Laramide districts and showing both alteration types. Alteration has apparently mobilized copper and other metals from diabase. The freshest diabase samples average about 120 ppm copper with little variation. In chloritic alteration, about 100 ppm of this copper is expelled in the most completely altered rocks. In actinolitic alteration, diabase may either gain or lose copper during alteration. Chloritic alteration constitutes roughly 70 percent of the diabase alteration in the study area, where alteration averages 41 percent complete. This implies liberation of about 9 ?? 106 tons (t) copper from diabase alteration, significantly less than the 16 ?? 106 t copper in Laramide mineral deposits of the superdistrict (Ray, Superior, Chilito, Christmas). However, diabase alteration may have been a significant component of the supply of copper to the Laramide mineral districts of the area. Synmineral magmatic sources of copper are not documented in this area. The distribution of Proterozoic diabase coincides with the central part of the southeastern Arizona copper province, which may thus

  2. Talking Rocks.

    ERIC Educational Resources Information Center

    Rice, Dale; Corley, Brenda

    1987-01-01

    Discusses some of the ways that rocks can be used to enhance children's creativity and their interest in science. Suggests the creation of a dramatic production involving rocks. Includes basic information on sedimentary, igneous, and metamorphic rocks. (TW)

  3. A reassessment of the early archaeological record at Leang Burung 2, a Late Pleistocene rock-shelter site on the Indonesian island of Sulawesi.

    PubMed

    Brumm, Adam; Hakim, Budianto; Ramli, Muhammad; Aubert, Maxime; van den Bergh, Gerrit D; Li, Bo; Burhan, Basran; Saiful, Andi Muhammad; Siagian, Linda; Sardi, Ratno; Jusdi, Andi; Abdullah; Mubarak, Andi Pampang; Moore, Mark W; Roberts, Richard G; Zhao, Jian-Xin; McGahan, David; Jones, Brian G; Perston, Yinika; Szabó, Katherine; Mahmud, M Irfan; Westaway, Kira; Jatmiko; Saptomo, E Wahyu; van der Kaars, Sander; Grün, Rainer; Wood, Rachel; Dodson, John; Morwood, Michael J

    2018-01-01

    This paper presents a reassessment of the archaeological record at Leang Burung 2, a key early human occupation site in the Late Pleistocene of Southeast Asia. Excavated originally by Ian Glover in 1975, this limestone rock-shelter in the Maros karsts of Sulawesi, Indonesia, has long held significance in our understanding of early human dispersals into 'Wallacea', the vast zone of oceanic islands between continental Asia and Australia. We present new stratigraphic information and dating evidence from Leang Burung 2 collected during the course of our excavations at this site in 2007 and 2011-13. Our findings suggest that the classic Late Pleistocene modern human occupation sequence identified previously at Leang Burung 2, and proposed to span around 31,000 to 19,000 conventional 14C years BP (~35-24 ka cal BP), may actually represent an amalgam of reworked archaeological materials. Sources for cultural materials of mixed ages comprise breccias from the rear wall of the rock-shelter-remnants of older, eroded deposits dated to 35-23 ka cal BP-and cultural remains of early Holocene antiquity. Below the upper levels affected by the mass loss of Late Pleistocene deposits, our deep-trench excavations uncovered evidence for an earlier hominin presence at the site. These findings include fossils of now-extinct proboscideans and other 'megafauna' in stratified context, as well as a cobble-based stone artifact technology comparable to that produced by late Middle Pleistocene hominins elsewhere on Sulawesi.

  4. Earth history. Low mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals.

    PubMed

    Planavsky, Noah J; Reinhard, Christopher T; Wang, Xiangli; Thomson, Danielle; McGoldrick, Peter; Rainbird, Robert H; Johnson, Thomas; Fischer, Woodward W; Lyons, Timothy W

    2014-10-31

    The oxygenation of Earth's surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth's surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans. Copyright © 2014, American Association for the Advancement of Science.

  5. Mineral inclusions in diamonds from the Kelsey Lake Mine, Colorado, USA: Depleted Archean mantle beneath the Proterozoic Yavapai province

    NASA Astrophysics Data System (ADS)

    Schulze, Daniel J.; Coopersmith, Howard G.; Harte, Ben; Pizzolato, Lori-Ann

    2008-03-01

    Yavapai province. The mixed diamond inclusion populations from the State Line kimberlites appear to support models in which volumes of Wyoming Craton Archean mantle survive buried beneath Proterozoic continental crust. Such material may be mixed with eclogitic/lherzolitic regimes emplaced beneath or intermingled with the Archean rocks by Proterozoic subduction.

  6. Magmatism and underplating, a broadband seismic perspective on the Proterozoic tectonics of the Great Falls and Snowbird Tectonic Zones

    NASA Astrophysics Data System (ADS)

    Chen, Y.; Gu, Y. J.; Dokht, R.; Wang, R.

    2017-12-01

    The crustal and lithospheric structures beneath the Western Canada Sedimentary Basin (WCSB) and northern Montana contain vital records of the Precambrian tectonic development of Laurentia. In this study, we analyze the broadband seismic data recorded by the USArray and the most complete set of regional seismic networks to date near the WCSB. We adopt an integrated approach to investigate crustal structure and history, based primarily on P-to-S receiver functions but incorporate results from noise correlation functions, finite-frequency tomography and potential field measurements. In comparison with existing regional and global models, our stacked receiver functions show considerable improvements in the resolution of both Moho depth and Vp/Vs ratio. We identify major variations in Moho depth from the WCSB to the adjacent Cordillera. The Moho deepens steeply from 40 km in the Alberta basin to 50 km beneath the foothills, following Airy isostasy, but thermal buoyancy may be responsible for a flat, shallow ( 35 km) Moho to the west of the Rocky Mountain Trench. The Moho depth also increases sharply near the Snowbird Tectonic Zone (STZ), which is consistent with earlier findings from active-source data. Multiple lower crustal phases, a high velocity shallow mantle and elevated Vp/Vs ratios along the westernmost STZ jointly suggest major Proterozoic subduction and magmatism along this collisional boundary. In northern Montana, the Moho deepens along the Great Falls Tectonic Zone (GFTZ), a proposed Proterozoic suture between the Medicine Hat Block and Wyoming craton. This transition occurs near the Little Belt Mountain, which is located south of the Great Falls Shear Zone, an extensive northeast striking fault system characterized by strong potential field gradients. Similar to the STZ, our receiver functions offer new evidence for Proterozoic underplating in the vicinity of the GFTZ. In view of similar rock ages near the collisional boundaries in all parts of northern

  7. Geochronology and geochemistry of late Paleozoic-early Mesozoic igneous rocks of the Erguna Massif, NE China: Implications for the early evolution of the Mongol-Okhotsk tectonic regime

    NASA Astrophysics Data System (ADS)

    Li, Yu; Xu, Wen-Liang; Wang, Feng; Tang, Jie; Zhao, Shuo; Guo, Peng

    2017-08-01

    We undertook geochemical and geochronological studies on late Paleozoic-early Mesozoic igneous rocks from the Erguna Massif with the aim of constraining the early evolution of the Mongol-Okhotsk tectonic regime. Zircon crystals from nine representative samples are euhedral-subhedral, display oscillatory growth zoning, and have Th/U values of 0.14-6.48, indicating a magmatic origin. U-Pb dating of zircon using SIMS and LA-ICP-MS indicates that these igneous rocks formed during the Late Devonian (∼365 Ma), late Carboniferous (∼303 Ma), late Permian (∼256 Ma), and Early-Middle Triassic (246-238 Ma). The Late Devonian rhyolites, together with coeval A-type granites, formed in an extensional environment related to the northwestwards subduction of the Heihe-Nenjiang oceanic plate. Their positive εHf(t) values (+8.4 to +14.4) and Hf two-stage model ages (TDM2 = 444-827 Ma) indicate they were derived from a newly accreted continental crustal source. The late Carboniferous granodiorites are geochemically similar to adakites, and their εHf(t) values (+10.4 to +12.3) and Hf two-stage model ages (TDM2 = 500-607 Ma) suggest they were sourced from thickened juvenile lower crustal material, this thickening may be related to the amalgamation of the Erguna-Xing'an and Songnen-Zhangguangcai Range massifs. Rocks of the late Permian to Middle Triassic suite comprise high-K calc-alkaline monzonites, quartz monzonites, granodiorites, and monzogranites. These rocks are relatively enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements. They were emplaced, together with coeval porphyry-type ore deposits, along an active continental margin where the Mongol-Okhotsk oceanic plate was subducting beneath the Erguna Massif.

  8. Cretaceous crust beneath SW Borneo: U-Pb dating of zircons from metamorphic and granitic rocks

    NASA Astrophysics Data System (ADS)

    Davies, L.; Hall, R.; Armstrong, R.

    2012-12-01

    Metamorphic basement rocks from SW Borneo are undated but have been suggested to be Palaeozoic. This study shows they record low pressure 'Buchan-type' metamorphism and U-Pb SHRIMP dating of zircons indicates a mid-Cretaceous (volcaniclastic) protolith. SW Borneo is the southeast promontory of Sundaland, the continental core of SE Asia. It has no sedimentary cover and the exposed basement has been widely assumed to be a crustal fragment from the Indochina-China margin. Metamorphic rocks of the Pinoh Group in Kalimantan (Indonesian Borneo) are intruded by granitoid rocks of Jurassic-Cretaceous age, based on K-Ar dating, suggesting emplacement mainly between 130 and 80 Ma. The Pinoh metamorphic rocks have been described as a suite of pelitic schists, slates, phyllites, and hornfelses, and have not been dated, although they have been correlated with rocks elsewhere in Borneo of supposed Palaeozoic age. Pelitic schists contain biotite, chlorite, cordierite, andalusite, quartz, plagioclase and in some cases high-Mn almandine-rich garnet. Many have a shear fabric associated with biotite and fibrolite intergrowth. Contact metamorphism due to intrusion of the granitoid rocks produced hornfelses with abundant andalusite and cordierite porphyroblasts. Granitoids range from alkali-granite to tonalite and contain abundant hornblende and biotite, with rare white mica. Zircons from granitoid rocks exhibit sector- and concentric- zoning; some have xenocrystic cores mantled by magmatic zircon. There are four important age populations at c. 112, 98, 84 and 84 Ma broadly confirming earlier dating studies. There is a single granite body with a Jurassic age (186 ± 2.3 Ma). Zircons from pelitic metamorphic rocks are typically euhedral, with no evidence of rounding or resorbing of grains; a few preserve volcanic textures. They record older ages than those from igneous rocks; U-Pb ages are Cretaceous with a major population between 134 and 110 Ma. A single sample contains Proterozoic

  9. White Rock

    NASA Technical Reports Server (NTRS)

    2002-01-01

    (Released 19 April 2002) The Science 'White Rock' is the unofficial name for this unusual landform which was first observed during the Mariner 9 mission in the early 1970's. As later analysis of additional data sets would show, White Rock is neither white nor dense rock. Its apparent brightness arises from the fact that the material surrounding it is so dark. Images from the Mars Global Surveyor MOC camera revealed dark sand dunes surrounding White Rock and on the floor of the troughs within it. Some of these dunes are just apparent in the THEMIS image. Although there was speculation that the material composing White Rock could be salts from an ancient dry lakebed, spectral data from the MGS TES instrument did not support this claim. Instead, the White Rock deposit may be the erosional remnant of a previously more continuous occurrence of air fall sediments, either volcanic ash or windblown dust. The THEMIS image offers new evidence for the idea that the original deposit covered a larger area. Approximately 10 kilometers to the southeast of the main deposit are some tiny knobs of similarly bright material preserved on the floor of a small crater. Given that the eolian erosion of the main White Rock deposit has produced isolated knobs at its edges, it is reasonable to suspect that the more distant outliers are the remnants of a once continuous deposit that stretched at least to this location. The fact that so little remains of the larger deposit suggests that the material is very easily eroded and simply blows away. The Story Fingers of hard, white rock seem to jut out like icy daggers across a moody Martian surface, but appearances can be deceiving. These bright, jagged features are neither white, nor icy, nor even hard and rocky! So what are they, and why are they so different from the surrounding terrain? Scientists know that you can't always trust what your eyes see alone. You have to use other kinds of science instruments to measure things that our eyes can

  10. Proterozoic crustal evolution of the Eucla basement, Australia: Implications for destruction of oceanic crust during emergence of Nuna

    NASA Astrophysics Data System (ADS)

    Kirkland, C. L.; Smithies, R. H.; Spaggiari, C. V.; Wingate, M. T. D.; Quentin de Gromard, R.; Clark, C.; Gardiner, N. J.; Belousova, E. A.

    2017-05-01

    The crystalline basement beneath the Cretaceous to Cenozoic Bight and Eucla Basins, in Western Australia has received comparatively little attention even though it lies on the eastern margin of one of the most mineral resource endowed regions on the planet. This basement is characterized by a complex geological evolution spanning c. 2 billion years, but paucity of outcrop and younger basin cover present a daunting challenge to understand the basement geology. In this work the composition of the unexposed Proterozoic crystalline basement to the Bight and Eucla Basins is investigated through zircon Hf isotopes and whole rock geochemistry from new drillcore samples. This region includes two geophysically defined basement entities: The Madura Province, containing: 1) c. 1478 Ma Sleeper Camp Formation, which has variable isotopic signatures including evolved values interpreted to reflect reworking of rare slivers of hyperextended Archean crust, 2) 1415-1389 Ma Haig Cave Supersuite, with mantle-like isotope values interpreted as melting of subduction-modified N-MORB source, and 3) 1181-1125 Ma Moodini Supersuite, with juvenile isotopic signatures interpreted to reflect mixed mafic lower-crustal and asthenospheric melts produced at the base of thinned crust. The Coompana Province, to the east of the Madura Province, has three major magmatic components: 1) c. 1610 Ma Toolgana Supersuite, with chemical and isotopic characteristics of primitive arc rock, 2) c. 1490 Ma Undawidgi Supersuite, with juvenile isotope values consistent with extensional processes involving asthenospheric input and 3) 1192-1140 Ma Moodini Supersuite, with strong isotopic similarity to Moodini Supersuite rocks in the Madura Province. This new isotopic and geochemical data shows that the Madura and Coompana regions together represent a huge tract of predominantly juvenile material. Magma sources recognised, include; 1) depleted mantle, producing MORB-like crust at c. 1950 Ma, but also contributing to

  11. Co-Cu-Au deposits in metasedimentary rocks-A preliminary report

    Slack, J.F.; Causey, J.D.; Eppinger, R.G.; Gray, J.E.; Johnson, C.A.; Lund, K.I.; Schulz, K.J.

    2010-01-01

    A compilation of data on global Co-Cu-Au deposits in metasedimentary rocks refines previous descriptive models for their occurrence and provides important information for mineral resource assessments and exploration programs. This compilation forms the basis for a new classification of such deposits, which is speculative at this early stage of research. As defined herein, the Co-Cu-Au deposits contain 0.1 percent or more by weight of Co in ore or mineralized rock, comprising disseminated to semi-massive Co-bearing sulfide minerals with associated Fe- and Cu-bearing sulfides, and local gold, concentrated predominantly within rift-related, siliciclastic metasedimentary rocks of Proterozoic age. Some deposits have appreciable Ag ? Bi ? W ? Ni ? Y ? rare earth elements ? U. Deposit geometry includes stratabound and stratiform layers, lenses, and veins, and (or) discordant veins and breccias. The geometry of most deposits is controlled by stratigraphic layering, folds, axial-plane cleavage, shear zones, breccias, or faults. Ore minerals are mainly cobaltite, skutterudite, glaucodot, and chalcopyrite, with minor gold, arsenopyrite, pyrite, pyrrhotite, bismuthinite, and bismuth; some deposits have appreciable tetrahedrite, uraninite, monazite, allanite, xenotime, apatite, scheelite, or molybdenite. Magnetite can be abundant in breccias, veins, or stratabound lenses within ore or surrounding country rocks. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally have abundant biotite or albite. Mesoproterozoic metasedimentary successions constitute the predominant geologic setting. Felsic and (or) mafic plutons are spatially associated with many deposits and at some localities may be contemporaneous with, and involved in, ore formation. Geoenvironmental data for the Blackbird mining district in central Idaho indicate that weathering of

  12. Mesozoic contractile and extensional structures in the Boyer Gap area, northern Dome Rock Mountains, Arizona

    SciT

    Boettcher, S.S.

    1993-04-01

    Mesozoic polyphase contractile and superposed ductile extensional structures affect Proterozoic augen gneiss, Paleozoic metasedimentary rocks, and Jurassic granitoids in the Boyer Gap area of the northern Dome Rock Mtns, W-central Arizona. The nappe-style contractile structures are preserved in the footwall of the Tyson Thrust shear zone, which is one of the structurally lowest thrust faults in the E-trending Jurassic and Cretaceous Maria fold and thrust belt. Contractile deformation preceded emplacement of Late Cretaceous granite (ca 80 Ma, U-Pb zircon) and some may be older than variably deformed Late Jurassic leucogranite. Specifically, detailed structural mapping reveals the presence of a km-scalemore » antiformal syncline that apparently formed as a result of superposition of tight to isoclinal, south-facing folds on an earlier, north-facing recumbent fold. The stratigraphic sequence of metamorphosed Paleozoic cratonal strata is largely intact in the northern Dome Rock Mtns, such that overturned and upright stratigraphic units can be distinguished. A third phase of folding in the Boyer Gap area is distinguished by intersection lineations that are folded obliquely across the hinges of open to tight, sheath folds. The axial planes of the sheet folds are subparallel to the mylonitic foliation in top-to-the-northeast extensional shear zones. The timing of ductile extensional structures in the northern Dome Rock is constrained by [sup 40]Ar/[sup 39]Ar isochron ages of 56 Ma and 48 Ma on biotite from mylonitic rocks in both the hanging wall and footwall of the Tyson Thrust shear zone. The two early phases of folding are the dominant mechanism by which shortening was accommodated in the Boyer Gap area, as opposed to deformation along discrete thrust faults with large offset. All of the ductile extensional structures are spectacularly displayed at an outcrop scale but are not of sufficient magnitude to obliterate the km-scale Mesozoic polyphase contractile

  13. Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

    NASA Astrophysics Data System (ADS)

    Harlan, Stephen S.; Geissman, John W.

    1998-07-01

    The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single-polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1°, I = -41.9° ( k: = 94, α95 = 3.3°, N = 21 sites) and a paleomagnetic pole at 21.1°S, 221.1°E, (K = 89, A95 = 3.4°). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47-1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5-1.4 Ga poles of the North American apparent polar wander path.

  14. Paleomagnetism of the Middle Proterozoic Electra Lake Gabbro, Needle Mountains, southwestern Colorado

    Harlan, S.S.; Geissman, J.W.

    1998-01-01

    The Electra Lake Gabbro is a small 1.435 Ga pluton that intrudes 1.7 to 1.6 Ga gneisses and schists of the Needle Mountains in southwestern Colorado. Paleomagnetic samples were collected from the main phases of the gabbro, diabase dikes, granite, and alaskite dikes that cut the gabbro and from a partially melted zone in gneiss along the southern margin of the pluton. Gabbro, diabase, and some melt zone samples have a single-polarity characteristic magnetization of northeast declination (D) and moderate negative inclination (I). Demagnetization behavior and rock magnetic characteristics indicate that the remanence is carried by nearly pure magnetite. After correction for the minor west dip of overlying Paleozoic strata, we obtain a mean direction of D = 32.1??, I = -41.9?? (k = 94, ??95 = 3.3??, N = 21 sites) and a paleomagnetic pole at 21.1?? S, 221.1 ??E, (K= 89, A95 = 3.4??). This pole is similar to poles from the Middle Proterozoic Belt Supergroup but is located at a higher southerly latitude than poles from other 1.47-1.44 Ga plutons from North America, most of which plot at equatorial latitudes. The reason for this discrepancy is not clear but may result from a combination of factors, including unrecognized tilting of the gabbro, the failure of this relatively small pluton to fully average paleosecular variation, and uncertainties in the overall reliability of other 1.5-1.4 Ga poles of the North American apparent polar wander path.

  15. Organically preserved microbial endoliths from the late Proterozoic of East Greenland

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Golubic, S.; Green, J.; Swett, K.

    1986-01-01

    Diverse microorganisms ranging from cyanobacteria to eukaryotic algae and fungi live endolithically within ooids, hardgrounds and invertebrate shells on the present-day sea floor. These organisms are involved in the mechanical destruction of carbonates, and are useful ecological indicators of water depth and pollution. The Phanerozoic history of microbial endoliths has been elucidated through the study of microborings (the trace fossils of endolithic microorganisms) and rare cellularly preserved individuals, but nothing was known of the possible Precambrian evolution of comparable microorganisms until Campbell documented the occurrence of microborings in late Proterozoic ooids from central East Greenland. We now report the discovery of large populations of organically preserved endolithic microorganisms in silicified pisolites from 700-800-Myr-old Limestone-Dolomite Series of East Greenland. This fossil assemblage is significant for three reasons: (1) It confirms the prediction that oolites, pisolites and hardgrounds--the substrates for pre-Phanerozoic endoliths--provide a hitherto poorly explored but rewarding set of environments into which the search for early microfossils must be broadened; (2) the assemblage is diverse, containing about 12 taxa of morphologically distinct and previously unknown endolithic cyanobacteria, plus associated epilithic and interstitial populations; and (3) at least six of the fossil populations are indistinguishable in morphology, pattern of development, reproductive biology and inferred ecology from distinctive cyanobacterial species that bore ooids today in the Bahama Banks.

  16. Origin of the Eumetazoa: testing ecological predictions of molecular clocks against the Proterozoic fossil record

    NASA Technical Reports Server (NTRS)

    Peterson, Kevin J.; Butterfield, Nicholas J.

    2005-01-01

    Molecular clocks have the potential to shed light on the timing of early metazoan divergences, but differing algorithms and calibration points yield conspicuously discordant results. We argue here that competing molecular clock hypotheses should be testable in the fossil record, on the principle that fundamentally new grades of animal organization will have ecosystem-wide impacts. Using a set of seven nuclear-encoded protein sequences, we demonstrate the paraphyly of Porifera and calculate sponge/eumetazoan and cnidarian/bilaterian divergence times by using both distance [minimum evolution (ME)] and maximum likelihood (ML) molecular clocks; ME brackets the appearance of Eumetazoa between 634 and 604 Ma, whereas ML suggests it was between 867 and 748 Ma. Significantly, the ME, but not the ML, estimate is coincident with a major regime change in the Proterozoic acritarch record, including: (i) disappearance of low-diversity, evolutionarily static, pre-Ediacaran acanthomorphs; (ii) radiation of the high-diversity, short-lived Doushantuo-Pertatataka microbiota; and (iii) an order-of-magnitude increase in evolutionary turnover rate. We interpret this turnover as a consequence of the novel ecological challenges accompanying the evolution of the eumetazoan nervous system and gut. Thus, the more readily preserved microfossil record provides positive evidence for the absence of pre-Ediacaran eumetazoans and strongly supports the veracity, and therefore more general application, of the ME molecular clock.

  17. Pennsylvanian and Early Permian paleogeography of east-central California: Implications for the shape of the continental margin and the timing of continental truncation

    NASA Astrophysics Data System (ADS)

    Stone, Paul; Stevens, Calvin H.

    1988-04-01

    Pennsylvanian and Early Permian paleogeographic features in east-central California include a southeast-trending carbonate shelf edge and turbidite basin that we infer paralleled a segment of the western margin of the North American continent. This segment of the continental margin was oblique to an adjoining segment on the north that trended southwestward across Nevada into easternmost California. We propose that the southeast-trending segment of the margin originated by tectonic truncation of the originally longer southwest-trending segment in Early or Middle Pennsylvanian to late Early Permian time, significantly earlier than a previously hypothesized Late Permian or Early Triassic continental truncation event. We interpret the truncating structure to have been a sinistral transform fault zone along which a continental fragment was removed and carried southeastward into the Caborca-Hermosillo region of northern Mexico, where it is now represented by exposures of Late Proterozoic and Paleozoic miogeoclinal rocks.

  18. Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age

    PubMed Central

    Johnston, D. T.; Wolfe-Simon, F.; Pearson, A.; Knoll, A. H.

    2009-01-01

    Molecular oxygen (O2) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580–550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O2 production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O2 budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe2+ rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms. PMID:19805080

  19. Anoxygenic photosynthesis modulated Proterozoic oxygen and sustained Earth's middle age.

    PubMed

    Johnston, D T; Wolfe-Simon, F; Pearson, A; Knoll, A H

    2009-10-06

    Molecular oxygen (O(2)) began to accumulate in the atmosphere and surface ocean ca. 2,400 million years ago (Ma), but the persistent oxygenation of water masses throughout the oceans developed much later, perhaps beginning as recently as 580-550 Ma. For much of the intervening interval, moderately oxic surface waters lay above an oxygen minimum zone (OMZ) that tended toward euxinia (anoxic and sulfidic). Here we illustrate how contributions to primary production by anoxygenic photoautotrophs (including physiologically versatile cyanobacteria) influenced biogeochemical cycling during Earth's middle age, helping to perpetuate our planet's intermediate redox state by tempering O(2) production. Specifically, the ability to generate organic matter (OM) using sulfide as an electron donor enabled a positive biogeochemical feedback that sustained euxinia in the OMZ. On a geologic time scale, pyrite precipitation and burial governed a second feedback that moderated sulfide availability and water column oxygenation. Thus, we argue that the proportional contribution of anoxygenic photosynthesis to overall primary production would have influenced oceanic redox and the Proterozoic O(2) budget. Later Neoproterozoic collapse of widespread euxinia and a concomitant return to ferruginous (anoxic and Fe(2+) rich) subsurface waters set in motion Earth's transition from its prokaryote-dominated middle age, removing a physiological barrier to eukaryotic diversification (sulfide) and establishing, for the first time in Earth's history, complete dominance of oxygenic photosynthesis in the oceans. This paved the way for the further oxygenation of the oceans and atmosphere and, ultimately, the evolution of complex multicellular organisms.

  20. Petrology and age of volcanic-arc rocks from the continental margin of the Bering Sea: implications for Early Eocene relocation of plate boundaries

    Davis, A.S.; Pickthorn, L.-B.G.; Vallier, T.L.; Marlow, M. S.

    1989-01-01

    Eocene volcanic flow and dike rocks from the Beringian margin have arc characteristics, implying a convergent history for this region during the early Tertiary. Chemical and mineralogical compositions are similar to those of modern Aleutian-arc lavas. They also resemble volcanic-arc compositions from western mainland Alaska, although greater chemical diversity and a stronger continental influence are observed in the Alaskan mainland rocks. Early Eocene ages of 54.4-50.2 Ma for the Beringian samples are well constrained by conventional K-Ar ages of nine plagioclase separates and by concordant 40Ar/39Ar incremental heating and total-fusion experiments. A concordant U-Pb zircon age of 53 Ma for the quartz-diorite dike is in good agreement with the K-Ar data. Plate motion studies of the North Pacific Ocean indicate more northerly directed subduction prior to the Tertiary and a continuous belt of arc-type volcanism extending from Siberia, along the Beringian margin, into mainland Alaska. Around 56 Ma (chron 25-24), subduction changed to a more westerly direction and subduction-related volcanism ceased for most of mainland Alaska. The increasingly oblique angle of convergence should have ended subduction along the Beringian margin as well. However, consistent ages of 54-50 Ma indicate a final pulse in arc-type magmatism during this period of plate adjustment. -from Authors

  1. Intrusive Rock Database for the Digital Geologic Map of Utah

    Nutt, C.J.; Ludington, Steve

    2003-01-01

    information. The information in the database is from a variety of sources, including geologic maps at scales ranging from 1:500,000 to 1:24,000, and thesis monographs. The references are shown twice: alphabetically and by region. The digital geologic map of Utah (Hintze and others, 2000) classifies intrusive rocks into only 3 categories, distinguished by age. They are: Ti, Tertiary intrusive rock; Ji, Upper to Middle Jurassic granite to quartz monzonite; and pCi, Early Proterozoic to Late Archean intrusive rock. Use of the tables provided in this report will permit selection and classification of those rocks by lithology and age. This database is a pilot study by the Survey and Analysis Project of the U.S. Geological Survey to characterize igneous rocks and link them to a digital map. The database, and others like it, will evolve as the project continues and other states are completed. We release this version now as an example, as a reference, and for those interested in Utah plutonic rocks.

  2. Experimental silicification of the extremophilic Archaea Pyrococcus abyssi and Methanocaldococcus jannaschii: applications in the search for evidence of life in early Earth and extraterrestrial rocks.

    PubMed

    Orange, F; Westall, F; Disnar, J-R; Prieur, D; Bienvenu, N; Le Romancer, M; Défarge, Ch

    2009-09-01

    Hydrothermal activity was common on the early Earth and associated micro-organisms would most likely have included thermophilic to hyperthermophilic species. 3.5-3.3 billion-year-old, hydrothermally influenced rocks contain silicified microbial mats and colonies that must have been bathed in warm to hot hydrothermal emanations. Could they represent thermophilic or hyperthermophilic micro-organisms and if so, how were they preserved? We present the results of an experiment to silicify anaerobic, hyperthermophilic micro-organisms from the Archaea Domain Pyrococcus abyssi and Methanocaldococcus jannaschii, that could have lived on the early Earth. The micro-organisms were placed in a silica-saturated medium for periods up to 1 year. Pyrococcus abyssi cells were fossilized but the M. jannaschii cells lysed naturally after the exponential growth phase, apart from a few cells and cell remains, and were not silicified although their extracellular polymeric substances were. In this first simulated fossilization of archaeal strains, our results suggest that differences between species have a strong influence on the potential for different micro-organisms to be preserved by fossilization and that those found in the fossil record represent probably only a part of the original diversity. Our results have important consequences for biosignatures in hydrothermal or hydrothermally influenced deposits on Earth, as well as on early Mars, as environmental conditions were similar on the young terrestrial planets and traces of early Martian life may have been similarly preserved as silicified microfossils.

  3. A reassessment of the early archaeological record at Leang Burung 2, a Late Pleistocene rock-shelter site on the Indonesian island of Sulawesi

    PubMed Central

    Hakim, Budianto; Ramli, Muhammad; Aubert, Maxime; van den Bergh, Gerrit D.; Li, Bo; Burhan, Basran; Saiful, Andi Muhammad; Siagian, Linda; Sardi, Ratno; Jusdi, Andi; Abdullah; Mubarak, Andi Pampang; Moore, Mark W.; Roberts, Richard G.; Zhao, Jian-xin; McGahan, David; Jones, Brian G.; Perston, Yinika; Szabó, Katherine; Mahmud, M. Irfan; Westaway, Kira; Jatmiko; Saptomo, E. Wahyu; van der Kaars, Sander; Grün, Rainer; Wood, Rachel; Dodson, John

    2018-01-01

    This paper presents a reassessment of the archaeological record at Leang Burung 2, a key early human occupation site in the Late Pleistocene of Southeast Asia. Excavated originally by Ian Glover in 1975, this limestone rock-shelter in the Maros karsts of Sulawesi, Indonesia, has long held significance in our understanding of early human dispersals into ‘Wallacea’, the vast zone of oceanic islands between continental Asia and Australia. We present new stratigraphic information and dating evidence from Leang Burung 2 collected during the course of our excavations at this site in 2007 and 2011–13. Our findings suggest that the classic Late Pleistocene modern human occupation sequence identified previously at Leang Burung 2, and proposed to span around 31,000 to 19,000 conventional 14C years BP (~35–24 ka cal BP), may actually represent an amalgam of reworked archaeological materials. Sources for cultural materials of mixed ages comprise breccias from the rear wall of the rock-shelter–remnants of older, eroded deposits dated to 35–23 ka cal BP–and cultural remains of early Holocene antiquity. Below the upper levels affected by the mass loss of Late Pleistocene deposits, our deep-trench excavations uncovered evidence for an earlier hominin presence at the site. These findings include fossils of now-extinct proboscideans and other ‘megafauna’ in stratified context, as well as a cobble-based stone artifact technology comparable to that produced by late Middle Pleistocene hominins elsewhere on Sulawesi. PMID:29641524

  4. Geological evolution of the late Proterozoic ``Mozambique Belt'' of Kenya

    NASA Astrophysics Data System (ADS)

    Mosley, P. N.

    1993-05-01

    Within the "Mozambique Belt" of Kenya at least four distinct tectonothermal episodes are recognised on Rb-Sr isotopics. The dates are in broad agreement with those from surrounding countries; principal ages/age ranges being 830 - 800, ~ 760, 630 - 580 and 560 - 520 Ma. All except the last attained at least upper amphibolite/granulite grade (with local melts). The first event was responsible for the primary transformation of an essentially sedimentary sequence to paragneisses with an initial near-horizontal fabric parallel to the compositional layering. Associated with the later part of the first phase, and linked to the second, is the emplacement of allochthonous ophiolitic and volcanosedimentary "packages", coupled with thrusting and imbrication of the paragneiss groups. The subsequent phases record progressive shortening across the orogenic belt during collision between two major continental fragments (east and west Gondwana), involving extensive structural reorganisation and isotopic resetting. During the progressive 630 - 580 Ma event, regional N-S- to NNW-SSE-trending ductile shear zones (generally sinistral) were produced giving the dominant regional fabric (including a regional N-S-stretching lineation), and controlling the present gross distribution of gneiss groups. Cooling and uplift post a ~ 560 Ma thermal event has exposed high-grade gneisses with a distinct structural and metamorphic asymmetry across the orogen. The western part of the orogen shows clockwise P- T- t paths and involves overthrusting of, and imbrication with, the Tanzanian craton which probably obscures older (1900 and 1100 Ma) tectonothermal episodes. In contrast, the eastern part has anti-clockwise P- T- t paths, is characterised by extensive crustal melts, and retains the isotopic imprint of earlier Proterozoic events. The present level of uplift exposes tectonised high-grade gneisses of more than one age. Current evidence supports the suggestion that low-grade ophiolitic

  5. A Modern Analogue for Proterozoic Inverse Carbon Isotope Signatures

    NASA Astrophysics Data System (ADS)

    Close, H. G.; Diefendorf, A. F.; Freeman, K. H.; Pearson, A.

    2008-12-01

    The carbon isotope distribution preserved in sedimentary lipids changes near the Neoproterozoic-Cambrian boundary. In older samples, n-alkyl lipids contain more 13C than both isoprenoid lipids and kerogen [1]. In younger samples, the opposite prevails. Although extreme heterotrophy has been invoked as a mechanism to explain the enrichment in 13C [2], here we suggest another explanation. The switch may reflect a fundamental transition from an oligotrophic ocean dominated by prokaryotic biomass, to an ocean in which carbon fixation is more intensive and burial is dominated by eukaryotic biomass. An analogue for Proterozoic ordering is found in the modern, oligotrophic Pacific Ocean, where n-alkyl lipids of picoplankton (0.2-0.5 μm particulate matter) contain excess 13C relative to the same lipids found in larger size classes (> 0.5 μm). Picoplanktonic lipids are heavier isotopically (-18 ‰) than both the sterols of eukaryotes (-23 ‰ to -26 ‰) and the total organic matter (-20 ‰; TOM). The 0.2-0.5 μm size class also has a distinct chain-length abundance profile. Although large particles must be the vehicle for total carbon export, paradoxically the lipid component of export production appears to be dominated by the 0.2-0.5 μm source. The picoplanktonic chain lengths and isotopic composition dominate lipids of TOM at 670 meters. When the ratio of prokaryotic to eukaryotic production is high, as in the modern central Pacific Ocean, it appears that exported material has an inverse carbon isotope signature similar to that preserved in Precambrian samples. [1] Logan, G. A. et al., Nature 376:53-56 (1995). [2] Rothman, D. H. et al., PNAS 100:8124-8129 (2003).

  6. Vent Complexes above Dolerite Sills in Phanerozoic LIPs: Implications for Proterozoic LIPs and IOCG Deposits

    NASA Astrophysics Data System (ADS)

    Ernst, R. E.; Bleeker, W.; Svensen, H.; Planke, S.; Polozov, A. G.

    2009-05-01

    New insights into the origin of IOCG (iron oxide copper gold) deposits [e.g., 1, 2, 3] follow from recent studies of Phanerozoic Large Igneous Provinces (LIPs). Detailed seismic studies of the 62-55 Ma North Atlantic Igneous Province and complementary studies in the 183 Ma Karoo and 250 Ma Siberian LIPs reveal thousands of hydrothermal vent complexes (HVCs). Up to 5-10 km across at the paleosurface, these vents connect to underlying dolerite sills at paleodepths of up to 8 km [4, 5, 6, 7]. They originate from explosive release of gases generated when thick sills (>50 m) are emplaced into volatile-rich but low-permeability sedimentary strata. HVCs are phreatomagmatic in origin. Their architecture, economic potential for IOCG-type deposits, and effects on climate strongly depend on the type of host rocks (black shales at Karoo and evaporites at Siberian LIPs) and its fluid (brines) saturation at the time of emplacement. About 250 HVCs associated with the Siberian LIP are mineralized having magnetite in the matrix. Some are being mined for Fe (Korshunovskoe and Rudnogorskoe), but their economic potential for copper and gold mineralization is understudied. These observations from the Phanerozoic LIP record suggest that HVCs should also be an essential component of sill provinces associated with Proterozoic LIPs, with a potential for causing major climatic shifts and IOCG-type deposits, particularly if the host sediments include substantial evaporites. Two examples are discussed here. The 725 Ma Franklin LIP covers 1.1 Mkm2 in northern Canada [8]; in the Minto Inlier of Victoria Island, this event comprises volcanics, sills, and breccia pipes [9, 10]. The breccia pipes appear identical to HVCs and, furthermore, the presence of evaporites in the host sediments of the Shaler Supergroup suggests (based on the Siberian trap example) the potential for IOCG-type mineralization. Could 1.59 Ga sills, as exemplified by the exposed Western Channel Diabase sills on the eastern

  7. The Martian Soil as a Geochemical Sink for Hydrothermally Altered Crustal Rocks and Mobile Elements: Implications of Early MER Results

    NASA Technical Reports Server (NTRS)

    Newsom, H. E.; Nelson, M. J.; Shearer, C. K.; Draper, D. S.

    2005-01-01

    Hydrothermal and aqueous alteration can explain some of the exciting results from the MER team s analyses of the martian soil, including the major elements, mobile elements, and the nickel enrichment. Published results from the five lander missions lead to the following conclusions: 1) The soil appears to be globally mixed and basaltic with only small local variations in chemistry. Relative to martian basaltic meteorites and Gusev rocks the soils are depleted in the fluid-mobile element calcium, but only slightly enriched to somewhat depleted in iron oxide. 2) The presence of olivine in the soils based on M ssbauer data argues that the soil is only partly weathered and is more akin to a lunar regolith than a terrestrial soil. 3) The presence of bromine along with sulfur and chlorine in the soils is consistent with addition of a mobile element component to the soil.

  8. Elvis to Eminem: quantifying the price of fame through early mortality of European and North American rock and pop stars.

    PubMed

    Bellis, Mark A; Hennell, Tom; Lushey, Clare; Hughes, Karen; Tocque, Karen; Ashton, John R

    2007-10-01

    Rock and pop stars are frequently characterised as indulging in high-risk behaviours, with high-profile deaths amongst such musicians creating an impression of premature mortality. However, studies to date have not quantified differences between mortality experienced by such stars and general populations. This study measures survival rates of famous musicians (n = 1064) from their point of fame and compares them to matched general populations in North America and Europe. We describe and utilise a novel actuarial survival methodology which allows quantification of excess post-fame mortality in pop stars. Individuals from North America and Europe performing on any album in the All-Time Top 1000 albums from the music genres rock, punk, rap, R&B, electronica and new age. From 3 to 25 years post fame, both North American and European pop stars experience significantly higher mortality (more than 1.7 times) than demographically matched populations in the USA and UK, respectively. After 25 years of fame, relative mortality in European (but not North American) pop stars begins to return to population levels. Five-year post-fame survival rates suggest differential mortality between stars and general populations was greater in those reaching fame before 1980. Pop stars can suffer high levels of stress in environments where alcohol and drugs are widely available, leading to health-damaging risk behaviour. However, their behaviour can also influence would-be stars and devoted fans. Collaborations between health and music industries should focus on improving both pop star health and their image as role models to wider populations.

  9. Elvis to Eminem: quantifying the price of fame through early mortality of European and North American rock and pop stars

    PubMed Central

    Bellis, Mark A; Hennell, Tom; Lushey, Clare; Hughes, Karen; Tocque, Karen; Ashton, John R

    2007-01-01

    Background Rock and pop stars are frequently characterised as indulging in high‐risk behaviours, with high‐profile deaths amongst such musicians creating an impression of premature mortality. However, studies to date have not quantified differences between mortality experienced by such stars and general populations. Objective This study measures survival rates of famous musicians (n = 1064) from their point of fame and compares them to matched general populations in North America and Europe. Design We describe and utilise a novel actuarial survival methodology which allows quantification of excess post‐fame mortality in pop stars. Participants Individuals from North America and Europe performing on any album in the All‐Time Top 1000 albums from the music genres rock, punk, rap, R&B, electronica and new age. Results From 3 to 25 years post fame, both North American and European pop stars experience significantly higher mortality (more than 1.7 times) than demographically matched populations in the USA and UK, respectively. After 25 years of fame, relative mortality in European (but not North American) pop stars begins to return to population levels. Five‐year post‐fame survival rates suggest differential mortality between stars and general populations was greater in those reaching fame before 1980. Conclusion Pop stars can suffer high levels of stress in environments where alcohol and drugs are widely available, leading to health‐damaging risk behaviour. However, their behaviour can also influence would‐be stars and devoted fans. Collaborations between health and music industries should focus on improving both pop star health and their image as role models to wider populations. PMID:17873227

  10. "Rock Garden"

    1997-10-14

    This false color composite image of the Rock Garden shows the rocks "Shark" and "Half Dome" at upper left and middle, respectively. Between these two large rocks is a smaller rock (about 0.20 m wide, 0.10 m high, and 6.33 m from the Lander) that was observed close-up with the Sojourner rover (see PIA00989). http://photojournal.jpl.nasa.gov/catalog/PIA00987

  11. Early paleozoic gabbro-amphibolites in the structure of the Bureya Terrane (eastern part of the Central Asian Fold Belt): First geochronological data and tectonic position

    NASA Astrophysics Data System (ADS)

    Smirnov, Yu. V.; Sorokin, A. A.; Kudryashov, N. M.

    2012-07-01

    Resulting from U-Pb geochronological study, it has been found that the gabbro-amphibolites composing the Bureya (Turan) Terrane in the eastern part of the Central Asian Fold Belt are Early Paleozoic (Early Ordovician; 455 ± 1.5 Ma) in age rather than Late Proterozoic as was believed earlier. The gabbro-amphibolites and associated metabasalts are close to tholeiites of the intraoceanic island arcs in terms of the geochemical properties. It is suggested that the tectonic block composed of these rocks was initially a seafloor fragment that divided the Bureya and Argun terranes in the Early Paleozoic and was later tectonically incorporated into the modern structure of the Bureya Terrane as a result of Late Paleozoic and Mesozoic events.

  12. Science Rocks!

    ERIC Educational Resources Information Center

    Prestwich, Dorothy; Sumrall, Joseph; Chessin, Debby A.

    2010-01-01

    It all began one Monday morning. Raymond could not wait to come to large group. In his hand, he held a chunk of white granite he had found. "Look at my beautiful rock!" he cried. The rock was passed around and examined by each student. "I wonder how rocks are made?" wondered one student. "Where do they come from?"…

  13. Rock Finding

    ERIC Educational Resources Information Center

    Rommel-Esham, Katie; Constable, Susan D.

    2006-01-01

    In this article, the authors discuss a literature-based activity that helps students discover the importance of making detailed observations. In an inspiring children's classic book, "Everybody Needs a Rock" by Byrd Baylor (1974), the author invites readers to go "rock finding," laying out 10 rules for finding a "perfect" rock. In this way, the…

  14. Rock Art

    ERIC Educational Resources Information Center

    Henn, Cynthia A.

    2004-01-01

    There are many interpretations for the symbols that are seen in rock art, but no decoding key has ever been discovered. This article describes one classroom's experiences with a lesson on rock art--making their rock art and developing their own personal symbols. This lesson allowed for creativity, while giving an opportunity for integration…

  15. Rock Games.

    ERIC Educational Resources Information Center

    Topal, Cathy Weisman

    1985-01-01

    Elementary school children are given cards containing specific criteria for doing one or two tasks: sorting or arranging rocks. Sorting tasks involve children in picking out rocks with particular characteristics, such as color or shape. In the arranging tasks children are asked to arrange rocks according to size or value. (RM)

  16. Collecting Rocks.

    ERIC Educational Resources Information Center

    Barker, Rachel M.

    One of a series of general interest publications on science topics, the booklet provides those interested in rock collecting with a nontechnical introduction to the subject. Following a section examining the nature and formation of igneous, sedimentary, and metamorphic rocks, the booklet gives suggestions for starting a rock collection and using…

  17. Quantifying the impact of early calcite cementation on the reservoir quality of carbonate rocks: A 3D process-based model

    NASA Astrophysics Data System (ADS)

    Hosa, Aleksandra; Wood, Rachel

    2017-06-01

    The reservoir properties of carbonate rocks are controlled by both deposition and diagenesis. The latter includes the early precipitation of calcite cements, which can exert a strong control on the evolution of subsequent diagenetic pathways. We quantify the impact of early marine cement growth in grainstones on evolving pore space by examining trends in the relationship between cementation and permeability using a 3D process-based model (Calcite3D). The model assumes varying proportions of polycrystalline and monocrystalline grain types, upon which we grow isopachous and syntaxial calcite cement types, respectively. We model two syntaxial cement shapes, compact and elongated, that approximate the geometries of typical rhombohedral calcite forms. Results demonstrate the effect of cement competition: an increasing proportion of monocrystalline grains creates stronger competition and a reduction in the impact of individual grains on final calcite cement volume and porosity. Isopachous cement is effective in closing pore throats and limiting permeability. We also show that the impact of syntaxial cement on porosity occlusion and therefore flow is highly dependent on monocrystalline grain location and the orientation of crystal axes. This demonstrates the importance of diagenetic overprint in controlling the evolution of rock properties, but also that this process can be essentially random. We also show that diagenesis alone can create notable heterogeneity in the permeability of carbonates. While Calcite3D is successful in modelling realistic changes in cement volumes and pore space morphology, modelled permeabilities (0.01 -30D) are above the range reported in reservoir grainstones due to the very high permeability of the initial synthetic sediment deposit (58.9D). Poroperm data generated by Calcite3D, however, exhibits a linear relationship between the logarithms of porosity and permeability with a high coefficient of determination, as observed in natural media.

  18. Life on an Island: Early Settlers off the Rock Bound Coast of Maine. Teaching with Historic Places.

    ERIC Educational Resources Information Center

    Hobbs-Olson, Laurie

    This lesson, based on National Register of Historic Places files, describes early settlers' lives on some of the approximately 5,000 islands off the coast of Maine. During the mid-18th century many of these islands began to be inhabited by settlers eager to take advantage of this interface between land and sea. The lesson discusses the Blue Duck…

  19. SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province

    Bickford, M.E.; Wooden, J.L.; Bauer, R.L.

    2006-01-01

    Interest in Paleoarchean to early Mesoarchean crust in North America has been sparked by the recent identification of ca. 3800-3500 Ma rocks on the northern margin of the Superior craton in the Assean Lake region of northern Manitoba and the Porpoise Cove terrane in northern Quebec. It has long been known that similarly ancient gneisses are exposed on the southern margin of the Superior craton in the Minnesota River Valley and in northern Michigan, but the ages of these rocks have been poorly constrained, because methods applied in the 1960s through late 1970s were inadequate to unravel the complexities of their thermotectonic history. Rocks exposed in the Minnesota River Valley include a complex of migmatitic granitic gneisses, schistose to gneissic amphibolite, metagabbro, and paragneisses. The best-known units are the Morton Gneiss and the Montevideo Gneiss. The complex of ancient gneisses is intruded by a major younger, weakly deformed granite body, the Sacred Heart granite. Regional geophysical anomalies that extend across the Minnesota River Valley have been interpreted as defining boundaries between distinct blocks containing the various gneissic units. New sensitive high-resolution ion microprobe (SHRIMP) U-Pb data from complex zircons yielded the following ages: Montevideo Gneiss near Montevideo, 3485 ?? 10 Ma, granodiorite intrusion, 3385 ?? 8 Ma; Montevideo Gneiss at Granite Falls, 3497 ?? 9 Ma, metamorphic event, 3300-3350 Ma, mafic intrusion, 3141 ?? 2 Ma, metamorphic overprint (rims), 2606 ?? 4 Ma; Morton Gneiss: 3524 ?? 9 Ma, granodiorite intrusion, 3370 ?? 8 Ma, metamorphic overprints (growth of rims), 3140 ?? 2 Ma and 2595 ?? 4 Ma; biotite-garnet paragneiss, 2619 ?? 20 Ma; and Sacred Heart granite, 2604 ?? 4 Ma. Zircons from a cordierite-bearing feldspar-biotite schist overlying the Morton Gneiss yielded well-defined age peaks at 3520, 3480, 3380, and 3140 Ma, showing detrital input from most of the older rock units; 2600 Ma rims on these zircons

  20. 2D seismic interpretation and characterization of the Hauterivian-Early Barremian source rock in Al Baraka oil field, Komombo Basin, Upper Egypt

    NASA Astrophysics Data System (ADS)

    Ali, Moamen; Darwish, M.; Essa, Mahmoud A.; Abdelhady, A.

    2018-03-01

    Komombo Basin is located in Upper Egypt about 570 km southeast of Cairo; it is an asymmetrical half graben and the first oil producing basin in Upper Egypt. The Six Hills Formation is of Early Cretaceous age and subdivided into seven members from base to top (A-G); meanwhile the B member is of Hauterivian-Early Barremian and it is the only source rock of Komombo Basin. Therefore, a detailed study of the SR should be carried out, which includes the determination of the main structural elements, thickness, facies distribution and characterization of the B member SR which has not been conducted previously in the study area. Twenty 2D seismic lines were interpreted with three vertical seismic profiles (VSP) to construct the depth structure-tectonic map on the top of the B member and to highlight the major structural elements. The interpretation of depth structure contour map shows two main fault trends directed towards the NW-SE and NE to ENE directions. The NW-SE trend is the dominant one, creating a major half-graben system. Also the depth values range from -8400 ft at the depocenter in the eastern part to -4800 ft at the shoulder of the basin in the northwestern part of the study area. Meanwhile the Isopach contour map of the B member shows a variable thickness ranging between 300 ft to 750 ft. The facies model shows that the B member SR is composed mainly of shale with some sandstone streaks. The B member rock samples were collected from Al Baraka-1 and Al Baraka SE-1 in the eastern part of Komombo Basin. The results indicate that the organic matter content (TOC) has mainly good to very good (1-3.36 wt %), The B member samples have HI values in the range 157-365 (mg HC/g TOC) and dominated by Type II/III kerogen, and is thus considered to be oil-gas prone based on Rock-Eval pyrolysis, Tmax values between 442° and 456° C therefore interpreted to be mature for hydrocarbon generation. Based on the measured vitrinite equivalent reflectance values, the B member SR

  1. Rocks of the Columbia Hills

    Squyres, S. W.; Arvidson, R. E.; Blaney, D.L.; Clark, B. C.; Crumpler, L.; Farrand, W. H.; Gorevan, S.; Herkenhoff, K. E.; Hurowitz, J.; Kusack, A.; McSween, H.Y.; Ming, D. W.; Morris, R.V.; Ruff, S.W.; Wang, A.; Yen, A.

    2006-01-01

    The Mars Exploration Rover Spirit has identified five distinct rock types in the Columbia Hills of Gusev crater. Clovis Class rock is a poorly sorted clastic rock that has undergone substantial aqueous alteration. We interpret it to be aqueously altered ejecta deposits formed by impacts into basaltic materials. Wishstone Class rock is also a poorly sorted clastic rock that has a distinctive chemical composition that is high in Ti and P and low in Cr. Wishstone Class rock may be pyroclastic or impact in origin. Peace Class rock is a sedimentary material composed of ultramafic sand grains cemented by significant quantities of Mg- and Ca-sulfates. Peace Class rock may have formed when water briefly saturated the ultramafic sands and evaporated to allow precipitation of the sulfates. Watchtower Class rocks are similar chemically to Wishstone Class rocks and have undergone widely varying degrees of near-isochemical aqueous alteration. They may also be ejecta deposits, formed by impacts into Wishstone-rich materials and altered by small amounts of water. Backstay Class rocks are basalt/trachybasalt lavas that were emplaced in the Columbia Hills after the other rock classes were, either as impact ejecta or by localized volcanic activity. The geologic record preserved in the rocks of the Columbia Hills reveals a period very early in Martian history in which volcanic materials were widespread, impact was a dominant process, and water was commonly present. Copyright 2006 by the American Geophysical Union.

  2. Early differentiation of the silicate Earth : new constraints from isotopic investigation of rocks from the lunar highlands

    NASA Astrophysics Data System (ADS)

    Boyet, M.; Carlson, R.; Borg, L.; Connelly, J.; Horan, M.

    2012-04-01

    The isotopic similarity in O, Mo, W, Si, and Fe between lunar and terrestrial samples suggests that the two planetary bodies were equilibrated in the energetic aftermath of the giant impact that gave birth to the Moon [1]. Coupled 142Nd-143Nd isotope systematics of lunar samples including both low-Ti and high-Ti mare basalts along with KREEP basalts have been used to constrain the age of crystallization of the lunar interior [2-5]. These studies show that the Sm-Nd system in the lunar mantle closed in the interval of 180-250 Ma after the beginning of solar system formation, depending on the model considered for lunar mantle differentiation (1 or 2 stage-model and initial lunar Sm/Nd ratio). Does this age represent the age of Moon formation? A prolonged lunar magma ocean (LMO) might be expected given the insulating effect of the thick plagioclase crust, so closure of the Sm-Nd system in the lunar mantle, particularly in a late stage LMO component like KREEP, might substantially post-date lunar formation. We have recently determined a new age of 4360±3 Ma for the ferroan anorthosite (FAN) 60025 using the 207Pb-206Pb, 147Sm-143Nd and 146Sm-142Nd isotope systems [6]. This study is the first in which a single sample of FAN yielded consistent ages from multiple isotope dating techniques, strongly suggesting that this age indicates the time at which the sample crystallized. In order to pursue the question of whether Moon formation occurred over 100 Ma after solar system formation, we have investigated a number of lunar rocks sampling the highland crust from both the FAN and the Mg-suite groups. Internal Sm-Nd isochron on the norite 77215 yields an age of 4296±20 Ma, in agreement with the young age determined on 60025. We will show that our new data obtained on the 146Sm-142Nd systematics of the lunar crust support the scenario of a relative young age for the Moon. Thus, these results offer a unique opportunity to better constrain the composition of the terrestrial

  3. Geochronology and tectonic significance of Middle Proterozoic granitic orthogneiss, North Qaidam HP/UHP terrane, Western China

    Mattinson, C.G.; Wooden, J.L.; Liou, J.G.; Bird, D.K.; Wu, C.L.

    2006-01-01

    Amphibolite-facies para- and orthogneisses near Dulan, in the southeast part of the North Qaidam terrane, enclose minor ultra-high pressure (UHP) eclogite and peridotite. Field relations and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Ion microprobe U-Pb and REE analyses of zircons from two granitic orthogneisses indicate magmatic crystallization at 927 ?? Ma and 921 ?? 7 Ma. Zircon rims in one of these samples yield younger ages (397-618 Ma) compatible with partial zircon recrystallization during in-situ Ordovician-Silurian eclogite-facies metamorphism previously determined from eclogite and paragneiss in this area. The similarity between a 2496 ?? 18 Ma xenocrystic core and 2.4-2.5 Ga zircon cores in the surrounding paragneiss suggests that the granites intruded the sediments or that the granite is a melt of the older basement which supplied detritus to the sediments. The magmatic ages of the granitic orthogneisses are similar to 920-930 Ma ages of (meta)granitoids described further northwest in the North Qaidam terrane and its correlative west of the Altyn Tagh fault, suggesting that these areas formed a coherent block prior to widespread Mid Proterozoic granitic magmatism. ?? Springer-Verlag 2006.

  4. Oceanic Pb-isotopic sources of proterozoic and paleozoic volcanogenic massive sulfide deposits on Prince of Wales Island and vicinity, southeastern Alaska

    Ayuso, Robert A.; Karl, Susan M.; Slack, John F.; Haeussler, Peter J.; Bittenbender, Peter E.; Wandless, Gregory A.; Colvin, Anna

    2005-01-01

    Volcanogenic massive sulfide (VMS) deposits on Prince of Wales Island and vicinity in southeastern Alaska are associated with Late Proterozoic through Cambrian volcanosedimentary rocks of the Wales Group and with Ordovician through Early Silurian felsic volcanic rocks of the Moira Sound unit (new informal name). The massive sulfide deposits in the Wales Group include the Big Harbor, Copper City, Corbin, Keete Inlet, Khayyam, Ruby Tuesday, and Stumble-On deposits, and those in the Moira Sound unit include the Barrier Islands, Moira Copper, Niblack, and Nichols Bay deposits. Pb-isotopic signatures were determined on sulfide minerals (galena, pyrite, chalcopyrite, pyrrhotite, and sphalerite) to constrain metal sources of the massive sulfides and for comparison with data for other deposits in the region. Except for the Ruby Tuesday deposit, galena is relatively rare in most of these deposits. Pb-isotopic signatures distinguish the mainly Cu+Zn±Ag±Au massive sulfide deposits in the Wales Group from the Zn+Cu±Ag±Au massive sulfide deposits in the Moira Sound unit. Among the older group of deposits, the Khayyam deposit has the widest variation in Pb-isotopic ratios (206Pb/204Pb=17.169–18.021, 207Pb/204Pb=15.341–15.499, 208Pb/204Pb=36.546–37.817); data for the other massive sulfide deposits in the Wales Group overlap the isotopic variations in the Khayyam deposit. Pb-isotopic ratios for both groups of deposits are lower than those on the average crustal Pbevolution curve (µ=9.74), attesting to a large mantle influence in the Pb source. All the deposits show no evidence for Pb evolution primarily in the upper or lower continental crust. Samples from the younger group of deposits have scattered Pb-isotopic compositions and plot as a broad band on uranogenic and thorogenic Pb diagrams. Data for these deposits overlap the trend for massive sulfide deposits in the Wales Group but extend to significantly more radiogenic Pb-isotopic values. Pb-isotopic ratios of

  5. Early cretaceous lower crustal reworking in NE China: insights from geochronology and geochemistry of felsic igneous rocks from the Great Xing'an range

    NASA Astrophysics Data System (ADS)

    Li, Yinglei; Liu, Huichuan; Huangfu, Pengpeng; He, Hongyun; Liu, Yongzheng

    2018-01-01

    This paper presents new zircon LA-ICP-MS U-Pb ages and whole-rock geochemical data for two granitic plutons and rhyolites of the Baiyingaolao Formation in the western Xing'an range (NE China). The two syenogranite granitic plutons yield identical zircon U-Pb age of 142 ± 1 Ma, and the Baiyingaolao rhyolites yield zircon U-Pb age of 138 ± 2 Ma. The granites contain some hornblendes, and show low Zr and Zr + Nb + Ce + Y contents, and low A/CNK (0.98-1.11), Mg# (6-55), and FeOT/MgO values. Rhyolite samples show similar geochemical characteristics with A/CNK of 0.99-1.10 and Mg# of 14-21. In combination with the high K2O contents (4.43-5.61 wt%) and negative correlations between P2O5 and SiO2, both the granites and rhyolites were classified as high-K calc-alkaline I-type granitoids. All samples give high zirconium saturation temperature of 794-964 °C with few initially inherited zircons, and belong to high-temperature I-type granitoids. They were generated by dehydration melting of biotite/muscovite from sub-alkaline meta-basalts in lower crust depth, leaving garnet, amphibole, and plagioclase as the major residual minerals. The syenogranites and rhyolites are likely formed in Mongol-Okhotsk oceanic subduction setting. Incorporating other lower crust-originated felsic rocks in Erguna and Xing'an massifs and Songliao basin, it is argued that lower crustal reworking is pronounced in NE China during Early Cretaceous.

  6. High-resolution (SIMS) versus bulk sulfur isotope patterns of pyrite in Proterozoic microbialites with diverse mat textures

    NASA Astrophysics Data System (ADS)

    Gomes, M. L.; Fike, D. A.; Bergmann, K.; Knoll, A. H.

    2015-12-01

    Sulfur (S) isotope signatures of sedimentary pyrite preserved in marine rocks provide a rich suite of information about changes in biogeochemical cycling associated with the evolution of microbial metabolisms and oxygenation of Earth surface environments. Conventionally, these S isotope records are based on bulk rock measurements. Yet, in modern microbial mat environments, S isotope compositions of sulfide can vary by up to 40‰ over a spatial range of ~ 1 mm. Similar ranges of S isotope variability have been found in Archean pyrite grains using both Secondary Ion Mass Spectrometry and other micro-analytical techniques. These micron-scale patterns have been linked to changes in rates of microbial sulfate reduction and/or sulfide oxidation, isotopic distillation of the sulfate reservoir due to microbial sulfate reduction, and post-depositional alteration. Fine-scale mapping of S isotope compositions of pyrite can thus be used to differentiate primary environmental signals from post-depositional overprinting - improving our understanding of both. Here, we examine micron-scale S isotope patterns of pyrite in microbialites from the Mesoproterozoic-Neoproterozoic Sukhaya Tunguska Formation and Neoproterozoic Draken Formation in order to explore S isotope variability associated with different mat textures and pyrite grain morphologies. A primary goal is to link modern observations of how sulfide spatial isotope distributions reflect active microbial communities present at given depths in the mats to ancient processes driving fine-sale pyrite variability in microbialites. We find large (up to 60‰) S isotope variability within a spatial range of less than 2.5cm. The micron-scale S isotope measurements converge around the S isotope composition of pyrite extracted from bulk samples of the same microbialites. These micron-scale pyrite S isotope patterns have the potential to reveal important information about ancient biogeochemical cycling in Proterozoic mat environments

  7. Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with late Proterozoic intracratonic troughs

    NASA Astrophysics Data System (ADS)

    Alvarez, Philippe

    1995-09-01

    During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, the Sangha aulacogen being the main structure of this type in the studied area. This transverse basin connects with other basins on the northern and south-western borders of the craton. During the Cryogenian, this network of basins was filled with fluvio-deltaic and lacustrine periglacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the northern edge of the craton, leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons. These facies are similar to those developed in the West Congolian Schisto-calcaire (shale-limestone) ramp succession. The North-Central African ramp succession (sediment slope) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Mathematical analysis of the bedding pattern yields a period of 29 30 days for the lunar month, a result which is in agreement with astrophysical evidence for this epoch (i.e. 650 Ma ago). Major subsidence and seismic activity on this gently sloping platform, associated with the proximity of the Sangha aulacogen, caused the triggering of carbonate turbidites and mass flow deposits. The proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Congolian and North-Central African ramps prograded northwards and southwards, respectively, into the Sangha aulacogen. The sea at that time was restricted to a long graben-like basin, while a remaining area of marine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in

  8. Evidence for a Neoproterozoic carbonate ramp on the northern edge of the Central African craton: relations with Late Proterozoic intracratonic troughs

    NASA Astrophysics Data System (ADS)

    Alvarez, Philippe

    During Late Proterozoic times, the Archaean Central African craton was affected by trough faulting which led to the formation of grabens, the Sangha aulacogen being the main structure of this type in the studied area. This transverse basin connects with other basins on the northern and south-western borders of the craton. During the Cryogenian, this network of basins was filled with fluvio-deltaic and lacustrine periglacial deposits. The glacio-eustatic transgression in Neoproterozoic III (end-Proterozoic) times flooded extensive areas of shelf on the northern edge of the craton, leading to the development of carbonate sedimentation in a broad outer shelf environment associated with nearshore barriers and evaporitic lagoons. These facies are similar to those developed in the West Congolian Schisto-calcaire (shale-limestone) ramp succession.The North-Central African ramp succession (sediment slope) contains an example of tidal rhythmites in vertical accretion, which occurs beneath the barrier deposits on the subtidal outer shelf. Mathematical analysis of the bedding pattern yields a period of 29-30 days for the lunar month, a result which is in agreement with astrophysical evidence for this epoch (i.e. 650Ma ago). Major subsidence and seismic activity on this gently sloping platform, associated with the proximity of the Sangha aulacogen, caused the triggering of carbonate turbidites and mass flow deposits. The proliferation of microbial mats under euphotic conditions on an extensive shelf led to the build-up of a carbonate platform. During early Neoproterozoic III times, the West Congolian and North-Central African ramps prograded northwards and southwards, respectively, into the Sangha aulacogen. The sea at that time was restricted to a long graben-like basin, while a remaining area of marine sedimentation persisted into the Palaeozoic. Thus the pattern of end-Proterozoic carbonate sedimentation on the borders of the Central African craton can be interpreted in

  9. Alkalic rocks and resources of thorium and associated elements in the Powderhorn District, Gunnison County, Colorado

    Olson, J.C.; Hedlund, D.C.

    1981-01-01

    Alkalic igneous rocks and related concentrations of thorium, niobium, rare-earth elements, titanium, and other elements have long been known in the Powderhorn mining district and have been explored intermittently for several decades. The deposits formed chiefly about 570 m.y. (million years) ago in latest Precambrian or Early Cambrian time. They were emplaced in lower Proterozoic (Proterozoic X) metasedimentary, metavolcanic, and plutonic rocks. The complex of alkalic rocks of Iron Hill occupies 31 km 2 (square kilometers) and is composed of pyroxenite, uncompahgrite, ijolite, nepheline syenite, and carbonatite, in order of generally decreasing age. Fenite occurs in a zone, in places more than 0.6 km (kilometer) wide, around a large part of the margin of the complex and adjacent to alkalic dikes intruding Precambrian host rock. The alkalic rocks have a radioactivity, chiefly due to thorium, greater than that of the surrounding Powderhorn Granite (Proterozoic X) and metamorphic rocks. The pyroxenite, uncompahgrite, ijolite, and nepheline syenite, which form more than 80 percent of the complex, have fairly uniform radioactivity. Radioactivity in the carbonatite stock, carbonatite dikes, and the carbonatite-pyroxenite mixed rock zone, however, generally exceeds that in the other rocks of the complex. The thorium concentrations in the Powderhorn district occur in six types of deposits: thorite veins, a large massive carbonatite body, carbonatite dikes, trachyte dikes, magnetite-ilmeniteperovskite dikes or segregations, and disseminations in small, anomalously radioactive plutons chiefly of granite or quartz syenite that are older than rocks of the alkalic complex. The highest grade thorium concentrations in the district are in veins that commonly occur in steeply dipping, crosscutting shear or breccia zones in the Precambrian rocks. They range in thickness from a centimeter or less to 5 m (meters) and are as much as 1 km long. The thorite veins are composed chiefly of

  10. Rock flows

    NASA Technical Reports Server (NTRS)

    Matveyev, S. N.

    1986-01-01

    Rock flows are defined as forms of spontaneous mass movements, commonly found in mountainous countries, which have been studied very little. The article considers formations known as rock rivers, rock flows, boulder flows, boulder stria, gravel flows, rock seas, and rubble seas. It describes their genesis as seen from their morphological characteristics and presents a classification of these forms. This classification is based on the difference in the genesis of the rubbly matter and characterizes these forms of mass movement according to their source, drainage, and deposit areas.

  11. Stratigraphy of the late Proterozoic Murdama Group, Saudi Arabia

    Greene, Robert C.

    1993-01-01

    The Murdama group probably was deposited in a back-arc basin on a continental platform bounded on the west by an active volcanic arc above an east-dipping subduction zone. The position of the subduction zone, which was active during most of the deposition in the Afif belt, is marked by a belt of gabbro and ultramafic rocks herein named the jabal Burqah belt. The subduction zone later stepped out to the southwest to the Nabitah belt, and Murdama strata were deposited in the Jabal Hadhah, Mistahjed, and smaller basins.

  12. A morphogram for silica-witherite biomorphs and its application to microfossil identification in the early earth rock record.

    PubMed

    Rouillard, J; García-Ruiz, J-M; Gong, J; van Zuilen, M A

    2018-05-01

    Archean hydrothermal environments formed a likely site for the origin and early evolution of life. These are also the settings, however, were complex abiologic structures can form. Low-temperature serpentinization of ultramafic crust can generate alkaline, silica-saturated fluids in which carbonate-silica crystalline aggregates with life-like morphologies can self-assemble. These "biomorphs" could have adsorbed hydrocarbons from Fischer-Tropsch type synthesis processes, leading to metamorphosed structures that resemble carbonaceous microfossils. Although this abiogenic process has been extensively cited in the literature and has generated important controversy, so far only one specific biomorph type with a filamentous shape has been discussed for the interpretation of Archean microfossils. It is therefore critical to precisely determine the full distribution in morphology and size of these biomorphs, and to study the range of plausible geochemical conditions under which these microstructures can form. Here, a set of witherite-silica biomorph synthesis experiments in silica-saturated solutions is presented, for a range of pH values (from 9 to 11.5) and barium ion concentrations (from 0.6 to 40 mmol/L BaCl 2 ). Under these varying conditions, a wide range of life-like structures is found, from fractal dendrites to complex shapes with continuous curvature. The size, spatial concentration, and morphology of the biomorphs are strongly controlled by environmental parameters, among which pH is the most important. This potentially limits the diversity of environments in which the growth of biomorphs could have occurred on Early Earth. Given the variety of the observed biomorph morphologies, our results show that the morphology of an individual microstructure is a poor criterion for biogenicity. However, biomorphs may be distinguished from actual populations of cellular microfossils by their wide, unimodal size distribution. Biomorphs grown by diffusion in silica gel can

  13. Hg concentrations from Late Triassic and Early Jurassic sedimentary rocks: first order similarities and second order depositional and diagenetic controls

    NASA Astrophysics Data System (ADS)

    Yager, J. A.; West, A. J.; Bergquist, B. A.; Thibodeau, A. M.; Corsetti, F. A.; Berelson, W.; Bottjer, D. J.; Rosas, S.

    2016-12-01

    Mercury concentrations in sediments have recently gained prominence as a potential tool for identifying large igneous province (LIP) volcanism in sedimentary records. LIP volcanism coincides with several mass extinctions during the Phanerozoic, but it is often difficult to directly tie LIP activity with the record of extinction in marine successions. Here, we build on mercury concentration data reported by Thibodeau et al. (Nature Communications, 7:11147, 2016) from the Late Triassic and Early Jurassic of New York Canyon, Nevada, USA. Increases in Hg concentrations in that record were attributed to Central Atlantic Magmatic Province (CAMP) activity in association with the end-Triassic mass extinction. We expand the measured section from New York Canyon and report new mercury concentrations from Levanto, Peru, where dated ash beds provide a discrete chronology, as well as St. Audrie's Bay, UK, a well-studied succession. We correlate these records using carbon isotopes and ammonites and find similarities in the onset of elevated Hg concentrations and Hg/TOC in association with changes in C isotopes. We also find second order patterns that differ between sections and may have depositional and diagenetic controls. We will discuss these changes within a sedimentological framework to further understand the controls on Hg concentrations in sedimentary records and their implications for past volcanism.

  14. Palaeoclimate significance of speleothems in crystalline rocks: a test case from the Late Glacial and early Holocene (Vinschgau, northern Italy)

    NASA Astrophysics Data System (ADS)

    Koltai, Gabriella; Cheng, Hai; Spötl, Christoph

    2018-03-01

    Partly coeval flowstones formed in fractured gneiss and schist were studied to test the palaeoclimate significance of this new type of speleothem archive on a decadal-to-millennial timescale. The samples encompass a few hundred to a few thousand years of the Late Glacial and the early Holocene. The speleothem fabric is primarily comprised of columnar fascicular optic calcite and acicular aragonite, both indicative of elevated Mg / Ca ratios in the groundwater. Stable isotopes suggest that aragonite is more prone to disequilibrium isotope fractionation driven by evaporation and prior calcite/aragonite precipitation than calcite. Changes in mineralogy are therefore attributed to these two internal fracture processes rather than to palaeoclimate. Flowstones formed in the same fracture show similar δ18O changes on centennial scales, which broadly correspond to regional lacustrine δ18O records, suggesting that such speleothems may provide an opportunity to investigate past climate conditions in non-karstic areas. The shortness of overlapping periods in flowstone growth and the complexity of in-aquifer processes, however, render the establishment of a robust stacked δ18O record challenging.

  15. Carboniferous and older carbonate rocks: Lithofacies, extent, and reservoir quality: Chapter CC in The oil and gas resource potential of the Arctic National Wildlife Refuge 1002 area, Alaska

    Dumoulin, Julie A.

    1999-01-01

    Carboniferous and older carbonate rocks are potential hydrocarbon reservoir facies for four plays in the 1002 area of the Arctic National Wildlife Refuge. These rocks include several units in the pre-Carboniferous basement and the Carboniferous Lisburne Group. Data from exploratory wells west of the 1002 area, outcrops south of the 1002 area, seismic lines, and well logs are synthesized herein to infer carbonate lithofacies, extent, and reservoir character beneath the northeastern Arctic coastal plain.A chiefly shallow-water basement carbonate succession of Late Proterozoic through Early Devonian age (Katakturuk Dolomite, Nanook Limestone, and Mount Copleston Limestone) is interpreted to be present beneath much of the south-central 1002 area; it reaches 3,700 m thick in outcrop and is the primary reservoir for the Deformed Franklinian Play. A more heterogeneous lithologic assemblage of uncertain age forms basement in the northwestern part of the 1002 area; well data define three subunits that contain carbonate intervals 5- 50 m thick. These strata are prospective reservoirs for the Undeformed Franklinian Play and could also be reservoirs for the Niguanak- Aurora Play. Regional lithologic correlations suggest a Cambrian-Late Proterozoic(?) age for subunits one and two, and a slightly younger, later Cambrian-Silurian age for subunit three. Seismic and well data indicate that subunit one overlies subunit two and is overlain by subunit three. The Mississippian and Pennsylvanian Lisburne Group, a predominantly carbonate platform succession as much as 1 km thick, is projected beneath the southernmost part of the 1002 area and is a potential reservoir for the Ellesmerian Thrust-belt and Niguanak-Aurora Plays.Carbonate rocks in the 1002 area probably retain little primary porosity but may have locally well developed secondary porosity. Measured reservoir parameters in basement carbonate strata are low (porosity generally ≤ 5%; permeability ≤ 0.2 md) but drill

  16. Lead isotope compositions as guides to early gold mineralization: The North Amethyst vein system, Creede district, Colorado

    Foley, Nora K.; Ayuso, Robert A.

    1994-01-01

    Pb isotope compositions from the late stage of the North Amethyst vein system and from the Bondholder and central and southern Creede mining districts are more radiogenic than the host volcanic rocks of the central cluster of the San Juan volcanic field. Our Pb isotope results indicate that early Au mineralization of the North Amethyst area may represent the product of an older and relatively local hydrothermal system distinct from that of the younger base metal and Ag mineralization found throughout the region. Fluids that deposited Au minerals may have derived their Pb isotope composition by a greater degree of interaction with shallow, relatively less radiogenic volcanic wall rocks. The younger, base metal and Ag-rich mineralization that overprints the Au mineralization in the North Amethyst area clearly has a more radiogenic isotopic signature, which implies that the later mineralization derived a greater component of its Pb from Proterozoic source rocks, or sediments derived from them.Paragenetically early sulfide-rich vein assemblages have the least radiogenic galenas and generally also have the highest Au contents. Thus, identification of paragenetically early vein assemblages with relatively unradiogenic Pb isotope compositions similar to those of the North Amethyst area provides an additional exploration tool for Au in the central San Juan Mountains area.

  17. Thermobarometric studies on the Levack Gneisses: Footwall rocks to the Sudbury Igneous Complex

    NASA Technical Reports Server (NTRS)

    James, R. S.; Peredery, W.; Sweeny, J. M.

    1992-01-01

    Granulite and amphibolite facies gneisses and migmatites of the Levack Gneiss Complex occupy a zone up to 8 km wide around the northern part of the Sudbury Igneous Complex (SIC). Orthopyroxene- and garnet-bearing tonalitic and semipelitic assemblages of granulite facies grade occur within 3 km of the SIC together with lenses of mafic and pyroxenitic rock compositions normally represented by an amphibole +/- cpx-rich assemblage; amphibolite facies assemblages dominate elsewhere in this terrain. These 2.711-Ga gneisses were introduced by (1) the Cartier Granite Batholith during late Archaean to early Proterozoic time and (2) the SIC, at 1.85 Ga, which produced a contact aureole 1-1.5 km wide in which pyroxene hornfelses are common within 200-300 m of the contact. A suite of 12 samples including both the opx-gt and amphibole-rich rock compositions have been studied. Garnets in the semipelitic gneisses are variably replaced by a plg-bio assemblage. Thermobarometric calculations using a variety of barometers and thermometers reported in the literature suggest that the granulite facies assemblages formed at depths in the 21-28 km range (6-8 kbar). Textures and mineral chemistry in the garnet-bearing semipelitic rocks indicate that this terrain underwent a second metamorphic event during uplift to depth in the 5-11 km range (2-3 kbar) and at temperatures as low as 500-550 C. This latter event is distinct from thermal recrystallization caused by the emplacement of the SIC; it probably represents metamorphism attributable to intrusion of the Cartier Granite Batholith. These data allow two interpretations for the crustal uplift of the Levack Gneisses: (1) The gneisses were tectonically uplifted prior to the Sudbury Event (due to intrusion of the Cartier Batholith); or (2) the gneisses were raised to epizonal levels as a result of meteorite impact at 1.85 Ga.

  18. Elemental and Sr-Nd isotopic geochemistry of Cretaceous to Early Paleogene granites and volcanic rocks in the Sikhote-Alin Orogenic Belt (Russian Far East): implications for the regional tectonic evolution

    NASA Astrophysics Data System (ADS)

    Zhao, Pan; Jahn, Bor-ming; Xu, Bei

    2017-09-01

    The Sikhote-Alin Orogenic Belt in Russian Far East is an important Late Mesozoic to Early Cenozoic accretionary orogen related to the subduction of the Paleo-Pacific Plate. This belt was generated by successive accretion of terranes made of accretionary prisms, turbidite basins and island arcs to the continental margin of northeastern Asia (represented by the Bureya-Jiamusi-Khanka Block) from Jurassic to Late Cretaceous. In order to study the tectonic and crustal evolution of this orogenic belt, we carried out zircon U-Pb dating, and whole-rock elemental and Sr-Nd isotopic analyses on granites and volcanic rocks from the Primorye region of southern Sikhote-Alin. Zircon dating revealed three episodes of granitoid emplacement: Permian, Early Cretaceous and Late Cretaceous to Early Paleogene. Felsic volcanic rocks (mainly rhyolite, dacite and ignimbrite) that overlay all tectonostratigraphic terranes were erupted during 80-57 Ma, postdating the accretionary process in the Sikhote-Alin belt. The Cretaceous-Paleogene magmatism represents the most intense tectonothermal event in the Sikhote-Alin belt. Whole-rock major and trace elemental data show arc-like affinity for granitoids and volcanic rocks, indicating that they were likely generated in a supra-subduction setting. Their initial 87Sr/86Sr ratios range from 0.7048 to 0.7114, and εNd(t) values vary from +1.7 to -3.8 (mostly < 0). Thus, the elemental and Sr-Nd isotopic data suggest that the felsic magmas were generated by partial melting of source rocks comprising mantle-derived juvenile component and recycled crustal component. In addition to the occurrence in the Sikhote-Alin orogenic belt, Cretaceous to Early Paleogene magmatic rocks are also widespread in NE China, southern Korean peninsula, Japanese islands and other areas of Russian Far East, particularly along the coastal regions of the Okhotsk and Bering Seas. These rocks constitute an extended magmatic belt along the continental margin of NE Asia. The

  19. Isotopic compositions of carbonates and organic carbon from upper Proterozoic successions in Namibia: stratigraphic variation and the effects of diagenesis and metamorphism

    NASA Technical Reports Server (NTRS)

    Kaufman, A. J.; Hayes, J. M.; Knoll, A. H.; Germs, G. J.

    1991-01-01

    The carbon isotope geochemistry of carbonates and organic carbon in the late Proterozoic Damara Supergroup of Namibia, including the Nama, Witvlei, and Gariep groups on the Kalahari Craton and the Mulden and Otavi groups on the Congo Craton, has been investigated as an extension of previous studies of secular variations in the isotopic composition of late Proterozoic seawater. Subsamples of microspar and dolomicrospar were determined, through petrographic and cathodoluminescence examination, to represent the "least-altered" portions of the rock. Carbon-isotopic abundances in these phases are nearly equal to those in total carbonate, suggesting that 13C abundances of late Proterozoic fine-grained carbonates have not been significantly altered by meteoric diagenesis, although 18O abundances often differ significantly. Reduced and variable carbon-isotopic differences between carbonates and organic carbon in these sediments indicate that isotopic compositions of organic carbon have been altered significantly by thermal and deformational processes, likely associated with the Pan-African Orogeny. Distinctive stratigraphic patterns of secular variation, similar to those noted in other, widely separated late Proterozoic basins, are found in carbon-isotopic compositions of carbonates from the Nama and Otavi groups. For example, in Nama Group carbonates delta 13C values rise dramatically from -4 to +5% within a short stratigraphic interval. This excursion suggests correlation with similar excursions noted in Ediacaran-aged successions of Siberia, India, and China. Enrichment of 13C (delta 13C> +5%) in Otavi Group carbonates reflects those in Upper Riphean successions of the Akademikerbreen Group, Svalbard, its correlatives in East Greenland, and the Shaler Group, northwest Canada. The widespread distribution of successions with comparable isotopic signatures supports hypotheses that variations in delta 13C reflect global changes in the isotopic composition of late

  20. Digital surfaces and hydrogeologic data for the Mesozoic through early Tertiary rocks in the Southeastern Coastal Plain in parts of Mississippi, Alabama, Georgia, South Carolina, and Florida

    Cannon, Debra M.; Bellino, Jason C.; Williams, Lester J.

    2012-01-01

    A digital dataset of hydrogeologic data for Mesozoic through early Tertiary rocks in the Southeastern Coastal Plain was developed using data from five U.S. Geological Survey (USGS) reports published between 1951 and 1996. These reports contain maps and data depicting the extent and elevation of the Southeast Coastal Plain stratigraphic and hydrogeologic units in Florida and parts of Mississippi, Alabama, Georgia, and South Carolina. The reports are: Professional Paper 1410-B (Renken, 1996), Professional Paper 1088 (Brown and others, 1979), Professional Paper 524-G (Applin and Applin, 1967), Professional Paper 447 (Applin and Applin, 1965), and Circular 91 (Applin, 1951). The digital dataset provides hydrogeologic data for the USGS Energy Resources Program assessment of potential reservoirs for carbon sequestration and for the USGS Groundwater Resource Program assessment of saline aquifers in the southeastern United States. A Geographic Information System (ArcGIS 9.3.1) was used to construct 33 digital (raster) surfaces representing the top or base of key stratigraphic and hydrogeologic units. In addition, the Geographic Information System was used to generate 102 geo-referenced scanned maps from the five reports and a geo-database containing structural and thickness contours, faults, extent polygons, and common features. The dataset also includes point data of well construction and stratigraphic elevations and scanned images of two geologic cross sections and a nomenclature chart.

  1. Paleoenvironmental reconstruction of the early Neolithic to middle Bronze Age Peña Larga rock shelter (Álava, Spain) from the small mammal record

    NASA Astrophysics Data System (ADS)

    Rofes, Juan; Zuluaga, Mari Cruz; Murelaga, Xabier; Fernández-Eraso, Javier; Bailon, Salvador; Iriarte, María José; Ortega, Luis Ángel; Alonso-Olazabal, Ainhoa

    2013-03-01

    The Peña Larga site, a rock shelter on the southern slopes of the Cantabrian cordillera (north Spain), is an archeological deposit covering nearly 4000 years, from the early Neolithic to the middle Bronze Age (Atlantic/Subboreal chronozones). It was used both as a household and as a stable, with a hiatus in the Chalcolithic when it was used as a collective sepulcher. Nearly twenty-eight thousand small vertebrate elements were recovered from its seven stratigraphic units, of which 2553 items were identified to the genus and/or species levels. The assemblage is composed of mammals, birds, reptiles, and amphibians. Of these, small mammals were used for paleoenvironmental reconstruction since they are very sensitive to climatic conditions, the sample sizes are large, and their preservation is good. Their distributions over time, measured in terms of relative abundance, serve as reliable proxies of habitat and climate change. The reconstruction of Peña Larga's past environments based on small mammals roughly coincides with the pollen and the amphibian/reptile records on the local scale, and with that of an ice core from Central Greenland on the global scale. This makes it a valuable tool for comparative purposes both in the regional and continental scales.

  2. Reproducing early Martian atmospheric carbon dioxide partial pressure by modeling the formation of Mg-Fe-Ca carbonate identified in the Comanche rock outcrops on Mars

    NASA Astrophysics Data System (ADS)

    Berk, Wolfgang; Fu, Yunjiao; Ilger, Jan-Michael

    2012-10-01

    The well defined composition of the Comanche rock's carbonate (Magnesite0.62Siderite0.25Calcite0.11Rhodochrosite0.02) and its host rock's composition, dominated by Mg-rich olivine, enable us to reproduce the atmospheric CO2partial pressure that may have triggered the formation of these carbonates. Hydrogeochemical one-dimensional transport modeling reveals that similar aqueous rock alteration conditions (including CO2partial pressure) may have led to the formation of Mg-Fe-Ca carbonate identified in the Comanche rock outcrops (Gusev Crater) and also in the ultramafic rocks exposed in the Nili Fossae region. Hydrogeochemical conditions enabling the formation of Mg-rich solid solution carbonate result from equilibrium species distributions involving (1) ultramafic rocks (ca. 32 wt% olivine; Fo0.72Fa0.28), (2) pure water, and (3) CO2partial pressures of ca. 0.5 to 2.0 bar at water-to-rock ratios of ca. 500 molH2O mol-1rock and ca. 5°C (278 K). Our modeled carbonate composition (Magnesite0.64Siderite0.28Calcite0.08) matches the measured composition of carbonates preserved in the Comanche rocks. Considerably different carbonate compositions are achieved at (1) higher temperature (85°C), (2) water-to-rock ratios considerably higher and lower than 500 mol mol-1 and (3) CO2partial pressures differing from 1.0 bar in the model set up. The Comanche rocks, hosting the carbonate, may have been subjected to long-lasting (>104 to 105 years) aqueous alteration processes triggered by atmospheric CO2partial pressures of ca. 1.0 bar at low temperature. Their outcrop may represent a fragment of the upper layers of an altered olivine-rich rock column, which is characterized by newly formed Mg-Fe-Ca solid solution carbonate, and phyllosilicate-rich alteration assemblages within deeper (unexposed) units.

  3. Art Rocks with Rock Art!

    ERIC Educational Resources Information Center

    Bickett, Marianne

    2011-01-01

    This article discusses rock art which was the very first "art." Rock art, such as the images created on the stone surfaces of the caves of Lascaux and Altimira, is the true origin of the canvas, paintbrush, and painting media. For there, within caverns deep in the earth, the first artists mixed animal fat, urine, and saliva with powdered minerals…

  4. Architectural elements from Lower Proterozoic braid-delta and high-energy tidal flat deposits in the Magaliesberg Formation, Transvaal Supergroup, South Africa

    NASA Astrophysics Data System (ADS)

    Eriksson, Patrick G.; Reczko, Boris F. F.; Jaco Boshoff, A.; Schreiber, Ute M.; Van der Neut, Markus; Snyman, Carel P.

    1995-06-01

    Three architectural elements are identified in the Lower Proterozoic Magaliesberg Formation (Pretoria Group, Transvaal Supergroup) of the Kaapvaal craton, South Africa: (1) medium- to coarse-grained sandstone sheets; (2) fine- to medium-grained sandstone sheets; and (3) mudrock elements. Both sandstone sheet elements are characterised by horizontal lamination and planar cross-bedding, with lesser trough cross-bedding, channel-fills and wave ripples, as well as minor desiccated mudrock partings, double-crested and flat-topped ripples. Due to the local unimodal palaeocurrent patterns in the medium- to coarse-grained sandstone sheets, they are interpreted as ephemeral braid-delta deposits, which were subjected to minor marine reworking. The predominantly bimodal to polymodal palaeocurrent trends in the fine- to medium-grained sandstone sheets are inferred to reflect high-energy macrotidal processes and more complete reworking of braid-delta sands. The suspension deposits of mudrocks point to either braid-delta channel abandonment, or uppermost tidal flat sedimentation. The depositional model comprises ephemeral braid-delta systems which debouched into a high-energy peritidal environment, around the margins of a shallow epeiric sea on the Kaapvaal craton. Braid-delta and tidal channel dynamics are inferred to have been similar. Fine material in the Magaliesberg Formation peritidal complexes indicates that extensive aeolian removal of clay does not seem applicable to this example of the early Proterozoic.

  5. Petrology of metabasic and peridotitic rocks of the Songshugou ophiolite, Qinling orogen, China

    NASA Astrophysics Data System (ADS)

    Belic, Maximilian; Hauzenberger, Christoph; Dong, Yunpeng

    2013-04-01

    The Proterozoic Songshugou ophiolite outcrops as a rootless nappe which was emplaced into the southern margin of the Qinling Group. It consists mainly of amphibolite facies metamafic and -ultramafic rocks. Trace element geochemistry and isotope composition show that the mafic rocks are mainly E-MORB and T-MORB metabasalts (Dong et al., 2008b). Within the ophiolite sequence, ultramafic rocks consist mainly of peridotites and serpentinites. Particularly, extremely fresh dunites and harzburgites, are found which do not display a conspicuous metamorphic overprint. The low CaO (<0.39 wt.%) and Al2O3 (<0.51 wt.%) as well as high MgO (41-48 wt.%) contents classify them as depleted non-fertile mantle rocks. Chromite is found as disseminated phase but can sometimes form massive chromite bands. The platinumgroup mineral Laurite (RuS2) could be identified as inclusion in chromites. Usually part of Ru is substituted by Os and Ir. The metamafic rocks consist of garnet, amphibole, symplectitic pyroxenes, ilmenite, apatite, ±zoisite, ±sphene and show a strong metamorphic overprint. Garnet contains numerous inclusions in the core but are nearly inclusion free at the rim. The cores have sometimes snowball textures indicating initially syndeformative growth. Pure albite and prehnite were found in the central parts of the garnets. In the outer portions, pargasitic amphibole, rutile and rarely glaukophane were found. The symplectitic pyroxenes are of diopsidic composition which enclose prehnite and not albite, as common in retrograde eclogitic rocks. Different stages of garnet breakdown to plagioclase and amphibole, from thin plagioclase rims surrounding the garnets to plagioclase rich pseudomorphs, can be observed in different samples. Based on the glaukophane inclusions and symplectitic pyroxenes a high pressure metamorphic event can be concluded. The garnet breakdown to plagioclase and the symplectites clearly indicate a rapid exhumation phase. The age of the metamorphic event is

  6. Opportunity Rocks!

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This high-resolution image captured by the Mars Exploration Rover Opportunity's panoramic camera shows in superb detail a portion of the puzzling rock outcropping that scientists are eagerly planning to investigate. Presently, Opportunity is on its lander facing northeast; the outcropping lies to the northwest. These layered rocks measure only 10 centimeters (4 inches) tall and are thought to be either volcanic ash deposits or sediments carried by water or wind. The small rock in the center is about the size of a golf ball.

  7. Hydrogeology and simulation of groundwater flow in fractured-rock aquifers of the Piedmont and Blue Ridge Physiographic Provinces, Bedford County, Virginia

    McCoy, Kurt J.; White, Bradley A.; Yager, Richard M.; Harlow, George E.

    2015-09-11

    A steady-state groundwater-flow simulation for Bedford County was developed to test the conceptual understanding of flow in the fractured-rock aquifers and to compute a groundwater budget for the four major drainages: James River, Smith Mountain and Leesville Lakes, Goose Creek, and Big Otter River. Model results indicate that groundwater levels mimic topography and that minimal differences in aquifer properties exist between the Proterozoic basement crystalline rocks and Late Proterozoic-Cambrian cover crystalline rocks. The Big Otter River receives 40.8 percent of the total daily groundwater outflow from fractured-rock aquifers in Bedford County; Goose Creek receives 25.8 percent, the James River receives 18.2 percent, and Smith Mountain and Leesville Lakes receive 15.2 percent. The remaining percentage of outflow is attributed to pumping from the aquifer (consumptive use).

  8. Volcaniclastic habitats for early life on Earth and Mars: A case study from ˜3.5 Ga-old rocks from the Pilbara, Australia

    NASA Astrophysics Data System (ADS)

    Westall, Frances; Foucher, Frédéric; Cavalazzi, Barbara; de Vries, Sjoukje T.; Nijman, Wouter; Pearson, Victoria; Watson, Jon; Verchovsky, Alexander; Wright, Ian; Rouzaud, Jean-Noel; Marchesini, Daniele; Anne, Severine

    2011-08-01

    Within the context of present and future in situ missions to Mars to investigate its habitability and to search for traces of life, we studied the habitability and traces of past life in ˜3.5 Ga-old volcanic sands deposited in littoral environments an analogue to Noachian environments on Mars. The environmental conditions on Noachian Mars (4.1-3.7 Ga) and the Early Archaean (4.0-3.3 Ga) Earth were, in many respects, similar: presence of liquid water, dense CO 2 atmosphere, availability of carbon and bio-essential elements, and availability of energy. For this reason, information contained in Early Archaean terrestrial rocks concerning habitable conditions (on a microbial scale) and traces of past life are of relevance in defining strategies to be used to identify past habitats and past life on Mars. One such example is the 3.446 Ga-old Kitty's Gap Chert in the Pilbara Craton, NW. Australia. This formation consists of volcanic sediments deposited in a coastal mudflat environment and is thus a relevant analogue for sediments deposited in shallow water environments on Noachian Mars. Two main types of habitat are represented, a volcanic (lithic) habitat and planar stabilized sediment surfaces in sunlit shallow waters. The sediments hosted small (<1 μm in size) microorganisms that formed colonies on volcanic particle surfaces and in pore waters within the volcanic sediments, as well as biofilms on stabilised sediment surfaces. The microorganisms included coccoids, filaments and rare rod-shaped organisms associated with microbial polymer (EPS). The preserved microbial community was apparently dominated by chemotrophic organisms but some locally transported filaments and filamentous mat fragments indicate that possibly photosynthetic mats formed nearby. Both microorganisms and sediments were silicified during very early diagenesis. There are no macroscopic traces of fossilised life in these volcanic sediments and sophisticated instrumentation and specialized sample

  9. Rock Garden

    NASA Technical Reports Server (NTRS)

    1997-01-01

    This false color composite image of the Rock Garden shows the rocks 'Shark' and 'Half Dome' at upper left and middle, respectively. Between these two large rocks is a smaller rock (about 0.20 m wide, 0.10 m high, and 6.33 m from the Lander) that was observed close-up with the Sojourner rover (see PIA00989).

    Mars Pathfinder is the second in NASA's Discovery program of low-cost spacecraft with highly focused science goals. The Jet Propulsion Laboratory, Pasadena, CA, developed and manages the Mars Pathfinder mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology (Caltech). The Imager for Mars Pathfinder (IMP) was developed by the University of Arizona Lunar and Planetary Laboratory under contract to JPL. Peter Smith is the Principal Investigator.

  10. "Don't Say Nothin' Bad about My Baby": A Re-evaluation of Women's Roles in the Brill Building Era of Early Rock 'n' Roll.

    ERIC Educational Resources Information Center

    Rohlfing, Mary E.

    1996-01-01

    Explores a period in American rock 'n' roll known as the Brill Building era. Argues that this marked the massive entry of women into rock 'n' roll, providing anglo Americans with their first taste of a female youth culture focused on sexuality and their first exposure to the voices and vernacular of young, African American women. (PA)

  11. Isotopic ages of rocks in the northern Front Range, Colorado

    Wilson, Anna B.; Bryant, Bruce

    2006-01-01

    These maps, and the tables that accompany them, are a compilation of isotopic age determinations of rocks and minerals in four 1:100,000 quadrangles in the northern and central Front Range, Colorado. Phanerozoic (primarily Tertiary and Cretaceous) age data are shown on one map; Proterozoic data are on the other. A sample location map is included for ease of matching specific localities and data in the tables to the maps. Several records in the tables were not included in the maps because either there were ambiguous dates or lack of location precluded accurate plotting.

  12. Proterozoic Bushveld-Vredefort catastrophe: Possible causes and consequences

    NASA Technical Reports Server (NTRS)

    Elston, W. E.; Twist, D.

    1988-01-01

    Bushveld Complex and Vredefort Dome are unique features, formed in close proximity during the same time interval, approximately 2 Ga. Both show evidence of catastrophic events in the shallow marine environment of the otherwise stable Kaapvaal Craton. Explanation by multiple impacts of an asteroid, brecciated by an inter-asteroidal collision and disintegrating in Earth's gravity field is supported by pseudotachylite, shatter cones, coesite, and stishovite at Vredefort but these shock phenomena were not found in the Bushveld Complex. The Bushveld Complex was formerly interpreted as a lopolith, a view incompatible with gravity, electrical resistivity, magnetic, and seismic-reflection data. It is outlined by five inward-dipping lobes of layered ultramafic-mafic plutonic rocks that partly coalesce to form a basin-like feature 400 km in diameter and 65,000 sq. km. in area, equivalent to a small lunar mare. The Bushveld Complex is orders of magnitudes larger than other proposed terrestrial impact structures and differs from them in important ways. Its principal members, in order of age, are Rooiberg Felsite, RLS, and Lebowa Granite. The Bushveld-Vredefort events occurred during the interval from neutral or reducing atmosphere to oxidizing atmosphere. This transition is usually related to the evolution of photosynthesizing organisms. If the impact hypothesis for Bushveld-Vredefort can be confirmed, it may represent a global catastrophe sufficient to contribute to environmental changes favoring aerobic photosynthesizing eukaryotes over anaerobic prokaryotes.

  13. Mantle xenoliths from Central Vietnam: evidence for at least Meso-Proterozoic formation of the lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Proßegger, Peter; Ntaflos, Theodoros; Ackerman, Lukáš; Hauzenberger, Christoph; Tran, Tuan Anh

    2016-04-01

    Intraplate Cenozoic basalts that are widely dispersed along the continental margin of East Asia belong to the Western Pacific "diffuse" igneous province. They consist mainly of alkali basalts, basanites,rarely nephelinites, which are mantle xenolith-bearing, potassic rocks and quartz tholeiites. The volcanism in this area has been attributed to the continental extension caused by the collision of India with Asia and by the subduction of the Pacific Ocean below Asia. We studied a suite of 24 mantle xenoliths from La Bang Lake, Dak Doa district and Bien Ho, Pleiku city in the Gia Province, Central Vietnam. They are predominantly spinel lherzolites (19) but spinel harburgites (3) and two garnet pyroxenites are present as well. The sizes of the xenoliths range from 5 to 40 cm in diameter with medium to coarse-grained protogranular textures. Whole rock major and trace element analyses display a wide range of compositions. The MgO concentration varies from 36.0 to 45.8 wt% whereas Al2O3 and CaO range from 0.63 to 4.36 wt% and from 0.52 to 4.21 wt% (with one sample having CaO of 6.63 wt%) respectively. Both CaO and Al2O3 positively correlate with MgO most likely indicating that the sampled rocks were derived from a common mantle source experienced variable degrees of partial melting. Mineral analyses show that the rock forming minerals are chemically homogeneous. The Fo contents of olivine vary between 89.2 and 91.2 and the Mg# of orthopyroxene and clinopyroxene range from 89 to 92 and 89 to 94 respectively. The range of Cr# for spinel is 0.06-0.26. Model calculations in both whole rock and clinopyroxenes show that lithospheric mantle underneath Central Vietnam experienced melt extractions that vary between 2-7, 12-15 and 20-30%. The majority of the primitive mantle-normalized whole rock and clinopyroxene REE patterns are parallel to each other indicating that clinopyroxene is the main repository of the trace elements. Clinopyroxenes are divided into two groups: group A

  14. Paleozoic Assemblage of the Northern Sierra Terrane: New Geochronology And Geochemical Data From the Stitching Late Devonian - Early Carboniferous Bowman Lake Batholith, and Associated Rocks

    NASA Astrophysics Data System (ADS)

    Powerman, V.; Hanson, R. E.; Girty, G.; Tretiakov, A.

    2016-12-01

    Previous study (Grove et al., 2008) of detrital zircon ages and the timing of magmatism within the Northern Sierra terrane (NST) suggest that it is exotic relative to western Laurentia, and link it to the Paleozoic Arctic Realm, Baltica and Caledonides. NST is a composite terrane in the North America Cordillera, consisting of four distinct allochthons, thrusted upon each other. As a first step towards the understanding of the origin and tectonic development of the NST we have undertaken the SHRIMP-RG U-Pb zircon dating of the rocks from granites, granodiorites, trondhjemites, tonalites and hypabyssal intrusions, composing the Bowman Lake batholith. The batholith stitches the allochthons of the NST and its crystallization age signifies the timing of juxtaposition SHRIMP-RG analyses from 14 samples yielded an age range of ca. 352-369 Ma, which overlaps the Devonian-Mississipian boundary and constrains the minimum age for amalgamation. Additionally, we have acquired multiple XRF data, favoring the island arc provenance of the Bowman Lake batholith Batholith. Previously proposed ties between NST and Robert Mountains allochthon seem unlikely because the latter was accreted onto the western miogeocline of Laurentia during the Late Dev.-Early Miss. while the NST was most probably still situated within the Arctic Realm. This work has been supported by the grant #14.Z50.31.0017 of the Government of the Russian Federation and by the Russian Foundation for Basic Research grant #15-55-10055. We are grateful to Stanford-USGS SHRIMP-RG center, and personally to Marty Grove and Elizabeth Miller.

  15. Microbial and Metabolic Diversity of the Alkaline Hot Springs of Paoha Island: A Late Archean and Proterozoic Ocean Analogue Environment.

    NASA Astrophysics Data System (ADS)

    Foster, I. S.; Demirel, C.; Hyde, A.; Motamedi, S.; Frantz, C. M.; Stamps, B. W.; Nunn, H. S.; Oremland, R. S.; Rosen, M.; Miller, L. G.; Corsetti, F. A.; Spear, J. R.

    2016-12-01

    Paoha Island formed 450 years ago within Mono Lake, California, as a result of magmatic activity in the underlying Long Valley Caldera. Previous studies of Paoha Island hot springs focused on the presence of novel organisms adapted to high levels of arsenic (114-138 µM). However, the microbial community structure, relationship with Mono Lake, and preservation potential of these communities remains largely unexplored. Here, we present water chemistry, 16S and 18S rRNA gene sequences, and metagenomic data for spring water and biofilms sampled on a recently exposed mudflat along the shoreline of Paoha Island. Spring waters were hypoxic, alkaline, and saline, had variable temperature (39-70 °C near spring sources) and high concentrations of arsenic, sulfide and reduced organic compounds. Thermodynamic modeling based on spring water chemistry indicated that sulfide and methane oxidation were the most energetically favorable respiratory metabolisms. 16S rRNA gene sequencing revealed distinct communities in different biofilms: red biofilms were dominated by arsenite-oxidizing phototrophs within the Ectothiorhodospiraceae, while OTUs most closely related to the cyanobacterial genus Arthrospira were present in green biofilms, as well as a large proportion of sequences assigned to sulfur-oxidizing bacteria. Metagenomic analysis identified genes related to arsenic resistance, arsenic oxidation/reduction, sulfur oxidation and photosynthesis. Eukaryotic rRNA gene sequencing analyses revealed few detectable taxa in spring biofilms and waters compared to Mono Lake; springs receiving splash from the lake were dominated by the alga Picocystis. The co-occurrence of hypoxia, high pH, and close proximity of anoxygenic and oxygenic phototrophic mats makes this site a potential Archean/Proterozoic analogue environment, but suggests that similar environments if preserved in the rock record, may not preserve evidence for community dynamics or the existence of photosynthetic metabolisms.

  16. Stromatolites of the Mescal Limestone (Apache Group, middle Proterozoic, central Arizona): taxonomy, biostratigraphy, and paleoenvironments

    NASA Technical Reports Server (NTRS)

    Bertrand-Sarfati, J.; Awramik, S. M.

    1992-01-01

    The 25- to 30-m-thick Algal Member of the Mescal Limestone (middle Proterozoic Apache Group) contains two distinct stromatolitic units: at the base, a 2- to 3-m-thick unit composed of columnar stromatolites and above, a thicker unit of stratiform and pseudocolumnar stromatolites. Columnar forms from the first unit belong to the Group Tungussia, and two new Forms are described: T. mescalita and T. chrysotila. Among the pseudocolumnar stromatolites of the thicker unit, one distinctive new taxon, Apachina henryi, is described. Because of the low stromatolite diversity, the biostratigraphic value of this assemblage is limited. The presence of Tungussia is consistent with the generally accepted isotopic age for the Apache Group of 1200 to 1100 Ma. The Mescal stromatolites do not closely resemble any other known Proterozoic stromatolites in the southwestern United States or northwestern Mexico. Analyses of sedimentary features and stromatolite growth forms suggest deposition on a stable, flat, shallow, subtidal protected platform during phases of Tungussia growth. Current action probably influenced the development of columns, pseudocolumns, and elongate stromatolitic ridges; these conditions alternated with phases of relatively quiet water characterized by nonoriented stromatolitic domes and stratiform stromatolites. Stable conditions favorable for development of the Mescal stromatolites were short-lived and did not permit the development of thick, stromatolite-bearing units such as those characteristic of many Proterozoic sequences elsewhere.

  17. Diamictite from Nimrod Glacier area, Antarctica: Possible Proterozoic glaciation on the seventh continent

    NASA Astrophysics Data System (ADS)

    Stump, Edmund; Miller, Julia M. G.; Korsch, Russell J.; Edgerton, David G.

    1988-03-01

    Late Proterozoic glacial deposits have been found on all continents except Antarctica. Here we describe four units of Late Proterozoic diamictite, with a total thickness of about 10m, from Panorama Point, Nimrod Glacier area, Antarctica, which have characteristics compatible with glaciogenic origin. The diamictite occurs within the Goldie Formation, a sequence of marine turbidites, and is associated with a unit of mafic pillow lavas. The diamictite is commonly structureless and in places laminated. Coarse clasts occur as scattered pebbles and cobbles and as pebbly pods and beds. No striated or faceted clasts were found. A few pebbles may pierce the laminae, but a drop-stone origin is uncertain. Deformation and metamorphism have obscured subtleties of original sedimentary structure. Outsize clasts in laminated sandy siltstone (now schistose) suggest a glaciogenic origin for these diamictites, but deposition by mass-flow processes cannot be ruled out. The discovery in Antarctica of possible Late Proterozoic glaciogenic deposits extends their geographic distribution to all of the major continental masses.

  18. Paleobiology of distinctive benthic microfossils from the upper Proterozoic Limestone-Dolomite "Series," central East Greenland

    NASA Technical Reports Server (NTRS)

    Green, J. W.; Knoll, A. H.; Golubic, S.; Swett, K.

    1987-01-01

    Populations of Polybessurus bipartitus Fairchild ex Green et al., a large morphologically distinctive microfossil, occur in silicified carbonates of the Upper Proterozoic (700-800 Ma) Limestone-Dolomite "Series," central East Greenland. Large populations of well-preserved individuals permit reconstruction of P. bipartitus as a coccoidal unicell that "jetted" upward from the sediment by the highly unidirectional secretion of extracellular mucopolysaccharide envelopes. Reproduction by baeocyte formation is inferred on the basis of clustered envelope stalks produced by small cells. Sedimentological evidence indicates that P. bipartitus formed surficial crusts locally within a shallow peritidal carbonate platform. Among living microorganisms a close morphological, reproductive, and behavioral counterpart to Polybessurus is provided by populations of an as yet underscribed cyanobacterium found in coastal Bahamian environments similar to those in which the Proterozoic fossils occur. In general morphology and "jetting" behavior, this population resembles species of the genus Cyanostylon, Geitler (1925), but reproduces via baeocyte formation. Polybessurus is but one of the more than two dozen taxa in the richly fossiliferous biota of the Limestone-Dolomite "Series." This distinctive population, along with co-occurring filamentous cyanobacteria and other microfossils, contributes to an increasingly refined picture of ecological heterogeneity in late Proterozoic oceans.

  19. Paleobiology of distinctive benthic microfossils from the upper Proterozoic Limestone-Dolomite "Series," central East Greenland.

    PubMed

    Green, J W; Knoll, A H; Golubic, S; Swett, K

    1987-01-01

    Populations of Polybessurus bipartitus Fairchild ex Green et al., a large morphologically distinctive microfossil, occur in silicified carbonates of the Upper Proterozoic (700-800 Ma) Limestone-Dolomite "Series," central East Greenland. Large populations of well-preserved individuals permit reconstruction of P. bipartitus as a coccoidal unicell that "jetted" upward from the sediment by the highly unidirectional secretion of extracellular mucopolysaccharide envelopes. Reproduction by baeocyte formation is inferred on the basis of clustered envelope stalks produced by small cells. Sedimentological evidence indicates that P. bipartitus formed surficial crusts locally within a shallow peritidal carbonate platform. Among living microorganisms a close morphological, reproductive, and behavioral counterpart to Polybessurus is provided by populations of an as yet underscribed cyanobacterium found in coastal Bahamian environments similar to those in which the Proterozoic fossils occur. In general morphology and "jetting" behavior, this population resembles species of the genus Cyanostylon, Geitler (1925), but reproduces via baeocyte formation. Polybessurus is but one of the more than two dozen taxa in the richly fossiliferous biota of the Limestone-Dolomite "Series." This distinctive population, along with co-occurring filamentous cyanobacteria and other microfossils, contributes to an increasingly refined picture of ecological heterogeneity in late Proterozoic oceans.

  20. Controls on Atmospheric O2: The Anoxic Archean and the Suboxic Proterozoic

    NASA Astrophysics Data System (ADS)

    Kasting, J. F.

    2015-12-01

    Geochemists have now reached consensus that the Archean atmosphere was mostly anoxic, that a Great Oxidation Event (GOE) occurred at around 2.5 Ga, and that the ensuing Proterozoic atmosphere was consistently oxidized [1,2]. Evidence for this broad-scale change in atmospheric composition comes from a variety of sources, most importantly from multiple sulfur isotopes [3,4]. The details of both the Archean and Proterozoic environments remain controversial, however, as does the underlying cause of the GOE. Evidence of 'whiffs' of oxygen during the Archean [5] now extend back as far as 3.0 Ga, based on Cr isotopes [6]. This suggests that O2 was being produced by cyanobacteria well before the GOE and that the timing of this event may have been determined by secular changes in O2 sinks. Catling et al. [7] emphasized escape of hydrogen to space, coupled with progressive oxidation of the continents and a concomitant decrease in the flux of reduced gases from metamorphism. But hydrogen produced by serpentinization of seafloor could also have been a controlling factor [8]. Higher mantle temperatures during the Archean should have resulted in thicker, more mafic seafloor and higher H2 production; decreasing mantle temperatures during the Proterozoic should have led to seafloor more like that of today and a corresponding decrease in H2 production, perhaps by enough to trigger the GOE. Once the atmosphere became generally oxidizing, it apparently remained that way during the rest of Earth's history. But O2 levels in the mid-Proterozoic could have been as low at 10-3 times the Present Atmospheric Level (PAL) [9]. The evidence, once again, is based on Cr isotopes. Possible mechanisms for maintaining such a 'suboxic' Proterozoic atmosphere will be discussed. Refs: 1. H. D. Holland, Geochim. Cosmochim. Acta 66, 3811 (2002). 2. H. D. Holland, Philosophical Transactions of the Royal Society B-Biological Sciences 361, 903 (Jun 29, 2006). 3. J. Farquhar, H. Bao, M. Thiemans, Science

  1. Scattering from Rock and Rock Outcrops

    DTIC Science & Technology

    2015-09-30

    Scattering from Rock and Rock Outcrops Derek R. Olson The Pennsylvania State University Applied Research Laboratory, P.O. Box 30 State...In terms of target detection and classification, scattering from exposed rock on the seafloor, (i.e., individual rocks and rock outcrops) presents...levels, and other statistical measures of acoustic scattering from rocks and rock outcrops is therefore critical. Unfortunately (and curiously

  2. Early Holocene (8.6 ka) rock avalanche deposits, Obernberg valley (Eastern Alps): Landform interpretation and kinematics of rapid mass movement

    PubMed Central

    Ostermann, Marc; Sanders, Diethard; Ivy-Ochs, Susan; Alfimov, Vasily; Rockenschaub, Manfred; Römer, Alexander

    2012-01-01

    In the Obernberg valley, the Eastern Alps, landforms recently interpreted as moraines are re-interpreted as rock avalanche deposits. The catastrophic slope failure involved an initial rock volume of about 45 million m³, with a runout of 7.2 km over a total vertical distance of 1330 m (fahrböschung 10°). 36Cl surface-exposure dating of boulders of the avalanche mass indicates an event age of 8.6 ± 0.6 ka. A 14C age of 7785 ± 190 cal yr BP of a palaeosoil within an alluvial fan downlapping the rock avalanche is consistent with the event age. The distal 2 km of the rock-avalanche deposit is characterized by a highly regular array of transverse ridges that were previously interpreted as terminal moraines of Late-Glacial. ‘Jigsaw-puzzle structure’ of gravel to boulder-size clasts in the ridges and a matrix of cataclastic gouge indicate a rock avalanche origin. For a wide altitude range the avalanche deposit is preserved, and the event age of mass-wasting precludes both runout over glacial ice and subsequent glacial overprint. The regularly arrayed transverse ridges thus were formed during freezing of the rock avalanche deposits. PMID:24966447

  3. Early Holocene (8.6 ka) rock avalanche deposits, Obernberg valley (Eastern Alps): Landform interpretation and kinematics of rapid mass movement.

    PubMed

    Ostermann, Marc; Sanders, Diethard; Ivy-Ochs, Susan; Alfimov, Vasily; Rockenschaub, Manfred; Römer, Alexander

    2012-10-15

    In the Obernberg valley, the Eastern Alps, landforms recently interpreted as moraines are re-interpreted as rock avalanche deposits. The catastrophic slope failure involved an initial rock volume of about 45 million m³, with a runout of 7.2 km over a total vertical distance of 1330 m (fahrböschung 10°). 36 Cl surface-exposure dating of boulders of the avalanche mass indicates an event age of 8.6 ± 0.6 ka. A 14 C age of 7785 ± 190 cal yr BP of a palaeosoil within an alluvial fan downlapping the rock avalanche is consistent with the event age. The distal 2 km of the rock-avalanche deposit is characterized by a highly regular array of transverse ridges that were previously interpreted as terminal moraines of Late-Glacial. 'Jigsaw-puzzle structure' of gravel to boulder-size clasts in the ridges and a matrix of cataclastic gouge indicate a rock avalanche origin. For a wide altitude range the avalanche deposit is preserved, and the event age of mass-wasting precludes both runout over glacial ice and subsequent glacial overprint. The regularly arrayed transverse ridges thus were formed during freezing of the rock avalanche deposits.

  4. Late Jurassic-Early Cretaceous episodic development of the Bangong Meso-Tethyan subduction: Evidence from elemental and Sr-Nd isotopic geochemistry of arc magmatic rocks, Gaize region, central Tibet, China

    NASA Astrophysics Data System (ADS)

    Zhang, Yu-Xiu; Li, Zhi-Wu; Yang, Wen-Guang; Zhu, Li-Dong; Jin, Xin; Zhou, Xiao-Yao; Tao, Gang; Zhang, Kai-Jun

    2017-03-01

    The Bangong Meso-Tethys plays a critical role in the development of the Tethyan realm and the initial elevation of the Tibetan Plateau. However, its precise subduction polarity, and history still remain unclear. In this study, we synthesize a report for the Late Jurassic-Early Cretaceous two-phase magmatic rocks in the Gaize region at the southern margin of the Qiangtang block located in central Tibet. These rocks formed during the Late Jurassic-earliest Cretaceous (161-142 Ma) and Early Cretaceous (128-106 Ma), peaking at 146 Ma and 118 Ma, respectively. The presence of inherited zircons indicates that an Archean component exists in sediments in the shallow Qiangtang crust, and has a complex tectonomagmatic history. Geochemical and Sr-Nd isotopic data show that the two-phase magmatic rocks exhibit characteristics of arc magmatism, which are rich in large-ion incompatible elements (LIIEs), but are strongly depleted in high field strength elements (HFSEs). The Late Jurassic-earliest Cretaceous magmatic rocks mixed and mingled among mantle-derived mafic magmas, subduction-related sediments, or crustally-derived felsic melts and fluids, formed by a northward and steep subduction of the Bangong Meso-Tethys ocean crust. The magmatic gap at 142-128 Ma marks a flat subduction of the Meso-Tethys. The Early Cretaceous magmatism experienced a magma MASH (melting, assimilation, storage, and homogenization) process among mantle-derived mafic magmas, or crustally-derived felsic melts and fluids, as a result of the Meso-Tethys oceanic slab roll-back, which triggered simultaneous back-arc rifting along the southern Qiangtang block margin.

  5. Gold-bearing fluvial and associated tidal marine sediments of Proterozoic age in the Mporokoso Basin, northern Zambia

    NASA Astrophysics Data System (ADS)

    Andrews-Speed, C. P.

    1986-07-01

    The structurally defined Mporokoso Basin contains up to 5000 m of continental and marine clastic sediments and minor silicic volcanics which together form the Mporokoso Group. These rocks overlie unconformably a basement of silicic-intermediate igneous rocks and accumulated within the interval 1830-1130 Ma. This sedimentological study was restricted to the eastern end of the basin and was part of an assessment of the potential for palaeoplacer gold in the Mporokoso Group. At the base of the Mporokoso Group, the Mbala Formation consists of 1000-1500 m of purple sandstones and conglomerates deposited in a braided-stream system overlain by 500-1000 m of mature quartz arenites deposited in a tidal marine setting. A general coarsening-upward trend exists within the fluvial sediments. Sandy, distal braided-stream facies passes upwards into more proximal conglomeratic facies. In proximal sections, poorly sorted conglomerates form the top of the coarsening-up sequence which is 500-700 m thick. The overlying fluvial sediments fine upwards. The tidal marine sandstones at the top of the Mbala Formation resulted from reworking of fluvial sediments during a marine transgression. Well-exposed sections with fluvial conglomerates were studied in detail. Individual conglomerate bodies form sheets extending for hundreds of metres downstream and at least one hundred metres across stream, with little sign of deep scouring or channelling. They are generally matrix-supported. The whole fluvial sequence is characterised by a paucity of mud or silt. These conglomerates were deposited by large velocity, sheet flows of water which transported a bed-load of pebbles and sand. Most fine material settling out from suspension was eroded by the next flow. The great lateral and vertical extent and the uniformity of the fluvial sediments suggest that the sediments accumulated over an unconfined alluvial plain and that the tectonic evolution of the source area was relatively continuous and not

  6. Stratigraphic Framework of Cambrian and Ordovician Rocks in the Appalachian Basin from Sequatchie County, Tennessee, through Eastern Kentucky, to Mingo County, West Virginia

    Ryder, Robert T.; Crangle, Robert D.; Repetski, John E.; Harris, Anita G.

    2008-01-01

    Cross section H-H' is the seventh in a series of restored cross sections constructed by the lead author to show the stratigraphic framework of Cambrian and Ordovician rocks in the Appalachian basin from Pennsylvania to Tennessee. The sections show complexly intertongued carbonate and siliciclastic lithofacies, marked thickness variations, key marker horizons, unconformities, stratigraphic nomenclature of the Cambrian and Ordovician sequence, and major faults that offset Proterozoic basement and overlying lower Paleozoic rocks. Several of the drill holes along the cross section have yielded a variety of whole and (or) fragmented conodont elements. The identifiable conodonts are used to differentiate strata of Late Cambrian, Early Ordovician, and Middle Ordovician age, and their conodont color alteration index (CAI) values are used to establish the thermal maturity of the sequence. Previous cross sections in this series are G-G', F-F', E-E', D-D', C-C', and B-B'. Many of these cross sections (B-B', C-C', D-D', and G-G') have been improved with the addition of gamma-ray log traces, converted to digital images, and made accessible on the Web.

  7. Planetary biology and microbial ecology. Biochemistry of carbon and early life

    NASA Technical Reports Server (NTRS)

    Margulis, L. (Editor); Nealson, K. H. (Editor); Taylor, I. (Editor)

    1983-01-01

    Experiments made with cyanobacteria, phototrophic bacteria, and methanogenic bacteria are detailed. Significant carbon isotope fractionation data is included. Taken from well documented extant microbial communities, this data provides a basis of comparison for isotope fractionation values measured in Archean and Proterozoic (preCambrian) rocks. Media, methods, and techniques used to acquire data are also described.

  8. Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa

    NASA Astrophysics Data System (ADS)

    Eriksson, Patrick G.; Reczko, Boris F. F.

    1998-09-01

    Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone-very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings.

  9. Why Was Silcrete Heat-Treated in the Middle Stone Age? An Early Transformative Technology in the Context of Raw Material Use at Mertenhof Rock Shelter, South Africa.

    PubMed

    Schmidt, Patrick; Mackay, Alex

    2016-01-01

    People heat treated silcrete during the Middle Stone Age (MSA) in southern Africa but the spatial and temporal variability of this practice remains poorly documented. This paucity of data in turn makes it difficult to interrogate the motive factors underlying the application of this technique. In this paper we present data on heat treatment of silcrete through the Howiesons Poort and post-Howiesons Poort of the rock shelter site Mertenhof, located in the Western Cape of South Africa. In contrast to other sites where heat treatment has been documented, distance to rock source at Mertenhof can be reasonably well estimated, and the site is known to contain high proportions of a diversity of fine grained rocks including silcrete, hornfels and chert at various points through the sequence. Our results suggest the prevalence of heat treatment is variable through the sequence but that it is largely unaffected by the relative abundance of silcrete prevalence. Instead there is a strong inverse correlation between frequency of heat treatment in silcrete and prevalence of chert in the assemblage, and a generally positive correlation with the proportion of locally available rock. While it is difficult to separate individual factors we suggest that, at Mertenhof at least, heat treatment may have been used to improve the fracture properties of silcrete at times when other finer grained rocks were less readily available. As such, heat treatment appears to have been a component of the MSA behavioural repertoire that was flexibly deployed in ways sensitive to other elements of technological organisation.

  10. Model photoautrophs isolated from a Proterozoic ocean analog - aerobic life under anoxic conditions

    NASA Astrophysics Data System (ADS)

    Hamilton, T. L.; de Beer, D.; Klatt, J.; Macalady, J.; Weber, M.; Lott, C.; Chennu, A.

    2016-12-01

    The 1-2 billion year delay before the final rise of oxygen at the end of the Proterozoic represents an important gap in our understanding of ancient biogeochemical cycling. Primary production fueled by sulfide-dependent anoxygenic photosynthesis, including the activity of metabolically versatile cyanobacteria, has been invoked as a mechanism for sustaining low atmospheric O2 throughout much of the Proterozoic. However, we understand very little about photoautotrophs that inhabit Proterozoic-like environments present on Earth today. Here we report on the isolation and characterization of a cyanobacterium and a green sulfur bacterium that are the dominant members of pinnacle mats in Little Salt Spring—a karst sinkhole in Florida with perennially low levels of dissolved oxygen and sulfide. The red pinnacle mats bloom in the anoxic basin of the sinkhole and receive light that is of very poor quality to support photosynthesis. Characterization of the isolates is consistent with observations of oxygenic and anoxygenic photosynthesis in situ—both organisms perform anoxygenic photosynthesis under conditions of very low light quality and quantity. Oxygenic photosynthesis by the cyanobacterium isolate is inhibited by the presence of sulfide and under optimal light conditions, rates of anoxygenic photosynthesis are nearly double that of oxygenic photosynthesis. The green sulfur bacterium is tolerant of oxygen and has a very low affinity for sulfide. In Little Salt Spring, oxygenic photosynthesis occurs for only four hours a day and the water column remains anoxic because of a continuous supply of sulfide. Isolation and characterization of these photoautotrophs combined with our high resolution microsensor data in situ highlight microbial biogeochemical cycling in this exceptional site where aerobic microorganisms persist in a largely anoxic ecosystem.

  11. Crystalline rocks of the Strawberry Lake area, Front Range, Colorado

    Young, Edward J.

    1991-01-01

    This report is a petrographic and geochemical study of the bedrock and a petrologic discussion based on felsic-mafic and silica-saturation ratios of the Strawberry Lake area. This volume is published as chapters A and B. These chapters are not available separatelyThe Strawberry lake area lies between the Continental Divide and Granby, Colorado, just north of Tabernash. It is underlain by Proterozoic rocks composed of biotite gneiss and two plutons-Boulder Creek Granodiorite of the Routt Plutonic Suite and Silver Plume Granite of the Berthoud Plutonic Suite. Relict enclaves of biotite gneiss are not uncommon in the Boulder Creek Granodiorite, in the Silver Plume Granite, and in the granitic enclaves in the biotite gneiss. Granitic and mafic enclaves in the Boulder Creek Granodiorite, granitic enclaves in the Silver Plume Granite and in the biotite gneiss, and a Tertiary andesite porphyry dike complete the rock types.

  12. Paleomagnetic poles and polarity zonation from the middle proterozoic belt supergroup, montana and Idaho

    NASA Astrophysics Data System (ADS)

    Elston, Donald P.; Bressler, Stephen L.

    1980-01-01

    Twelve paleomagnetic poles and a preliminary polarity zonation are reported from geologically mapped, stratigraphically controlled sections of the middle Proterozoic Belt Supergroup in western Montana and northern Idaho. Although gaps exist in the stratigraphic coverage, the lower Belt, Ravalli Group, and middle Belt carbonate appear to be largely if not entirely of normal polarity. A switch to reversed polarity takes place near the base of the overlying Missoula Group (base of Snowslip Fomration), and two comparatively narrow intervals of mixed polarity containing at least 18 reversals are found in the lower and middle Snowslip. Seven reversals, mostly widely spaced in stratigraphic position, have been found in overlying strata of the Missoula Group. Poles from strata of the Ravalli Group through Missoula Group are well defined and tightly clustered. They plot in the south-central Pacific Ocean and display only a small (~20°) southeast to northwest shift in pole path. Poles from Belt strata in the eastern basin plot sytematically east of poles from the more westerly collection sites. A counterclockwise shift in declination of 7°-10° is seen in strata from the easterly sites, which has given rise to a curving pole path that closely parallels the path for the westerly sites. This easterly path appears explainable as a consequence of a counterclokwise structural rotation of eastern Belt strata relative to western and central Belt strata produced during west ot east thrust faulting of late Mesozoic and early Cenozoic age. An additional counterclockwise rotation of the same magnitude exists between the northern and southern parts of the Montana fisturbed belt of the eastern Belt basin. Moreover, anomalously steep inclinations, not yet explained, have been reported from some of the strata sampled at the north end of the disturbed belt. The steep inclinations result in poles that plot distinctly east of poles reported from this study. The Belt pole path lies in a part

  13. Rafted Rock

    2016-11-09

    This area of Amazonis Planitia to the west of the large volcano Olympus Mons was once flooded with lava. A huge eruption flowed out across the relatively flat landscape. Sometimes called "flood basalt," the lava surface quickly cooled and formed a thin crust of solidified rock that was pushed along with the flowing hot liquid rock. Hills and mounds that pre-dated the flooding eruption became surrounded, forming obstructions to the relentless march of lava. In this image, these obstructions appeared to have poked up and sliced through the lava crust as the molten rock and crust moved together from west to east, over and past the stationary mounds. The result is a series of parallel grooves or channels with the obstructing mound remaining at the western end as the flow came to rest. From such images scientists can reconstruct the direction of the lava flow, potentially tracing it back to the source vent. http://photojournal.jpl.nasa.gov/catalog/PIA21204

  14. The role of microbial iron reduction in the formation of Proterozoic molar tooth structures

    NASA Astrophysics Data System (ADS)

    Hodgskiss, Malcolm S. W.; Kunzmann, Marcus; Poirier, André; Halverson, Galen P.

    2018-01-01

    Molar tooth structures are poorly understood early diagenetic, microspar-filled voids in clay-rich carbonate sediments. They are a common structure in sedimentary successions dating from 2600-720 Ma, but do not occur in rocks older or younger, with the exception of two isolated Ediacaran occurrences. Despite being locally volumetrically significant in carbonate rocks of this age, their formation and disappearance in the geological record remain enigmatic. Here we present iron isotope data, supported by carbon and oxygen isotopes, major and minor element concentrations, and total organic carbon and sulphur contents for 87 samples from units in ten different basins spanning ca. 1900-635 Ma. The iron isotope composition of molar tooth structures is almost always lighter (modal depletion of 2‰) than the carbonate or residue components in the host sediment. We interpret the isotopically light iron in molar tooth structures to have been produced by dissimilatory iron reduction utilising Fe-rich smectites and Fe-oxyhydroxides in the upper sediment column. The microbial conversion of smectite to illite results in a volume reduction of clay minerals (∼30%) while simultaneously increasing pore water alkalinity. When coupled with wave loading, this biogeochemical process is a viable mechanism to produce voids and subsequently precipitate carbonate minerals. The disappearance of molar tooth structures in the mid-Neoproterozoic is likely linked to a combination of a decrease in smectite abundance, a decline in the marine DIC reservoir, and an increase in the concentration of O2 in shallow seawater.

  15. The Role of Microbial Iron Reduction in the Formation of Proterozoic Molar Tooth Structures

    NASA Astrophysics Data System (ADS)

    Hodgskiss, M. S. W.; Kunzmann, M.; Halverson, G. P.; Poirier, A.

    2016-12-01

    Molar tooth structures are poorly understood early diagenetic, microspar-filled voids in clay-rich carbonate sediments. They are a common structure in sedimentary successions dating from 2600-720 Ma, but do not occur in rocks older or younger. Despite being volumetrically significant in carbonate rocks of this age, their formation and disappearance are poorly understood. Here, we present iron isotope data, supported by carbon and oxygen isotopes, major and minor element concentrations, and total organic carbon and pyrite contents for samples from ten regions spanning 1870-635 Ma. The iron isotopic composition of molar tooth structures is almost always lighter (modal depletion of 2‰) than the carbonate or siliciclastic components in the host sediment, whereas carbon isotopes are indistinguishable. We interpret the isotopically light iron in molar tooth structures to have been produced by microbial iron reduction utilising Fe-oxyhydroxides and smectites. The microbial conversion of smectite to illite results in a volume reduction of clay minerals ( 30%), while locally increasing pore water alkalinity. Therefore, this biogeochemical process is a viable mechanism to produce voids and subsequently precipitate carbonate minerals. The disappearance of molar tooth structures is likely linked to a combination of a decrease in smectite abundance, a decline in the marine DIC reservoir, and increase in the concentration of O2 in shallow seawater in the mid-Neoproterozoic.

  16. Devonian post-orogenic extension-related volcano-sedimentary rocks in the northern margin of the Tibetan Plateau, NW China: Implications for the Paleozoic tectonic transition in the North Qaidam Orogen

    NASA Astrophysics Data System (ADS)

    Qin, Yu; Feng, Qiao; Chen, Gang; Chen, Yan; Zou, Kaizhen; Liu, Qian; Jiao, Qianqian; Zhou, Dingwu; Pan, Lihui; Gao, Jindong

    2018-05-01

    The Maoniushan Formation in the northern part of the North Qaidam Orogen (NQO), NW China, contains key information on a Paleozoic change in tectonic setting of the NQO from compression to extension. Here, new zircon U-Pb, petrological, and sedimentological data for the lower molasse sequence of the Maoniushan Formation are used to constrain the timing of this tectonic transition. Detrital zircons yield U-Pb ages of 3.3-0.4 Ga with major populations at 0.53-0.4, 1.0-0.56, 2.5-1.0, and 3.3-2.5 Ga. The maximum depositional age of the Maoniushan Formation is well constrained by a youngest detrital zircon age of ∼409 Ma. Comparing these dates with geochronological data for the region indicates that Proterozoic-Paleozoic zircons were derived mainly from the NQO as well as the Oulongbuluk and Qaidam blocks, whereas Archean zircons were probably derived from the Oulongbuluk Block and the Tarim Craton. The ∼924, ∼463, and ∼439 Ma tectonothermal events recorded in this region indicate that the NQO was involved in the early Neoproterozoic assembly of Rodinia and early Paleozoic microcontinental convergence. A regional angular unconformity between Devonian and pre-Devonian strata within the NQO suggests a period of strong mountain building between the Oulongbuluk and Qaidam blocks during the Silurian, whereas an Early Devonian post-orogenic molasse, evidence of extensional collapse, and Middle to Late Devonian bimodal volcanic rocks and Carboniferous marine carbonate rocks clearly reflect long-lived tectonic extension. Based on these results and the regional geology, we suggest that the Devonian volcano-sedimentary rocks within the NQO were formed in a post-orogenic extensional setting similar to that of the East Kunlun Orogen, indicating that a major tectonic transition from compression to extension in these two orogens probably commenced in the Early Devonian.

  17. Uranium Isotope Compositions of Mid-Proterozoic Organic-rich Mudrocks: Evidence for an Episode of Increased Ocean Oxygenation at ca. 1.36 Ga and Evaluation of the Effect of Post-Depositional Hydrothermal Fluid Flow

    NASA Astrophysics Data System (ADS)

    Kendall, B.; Yang, S.; Lu, X.; Zhang, F.; Zheng, W.

    2016-12-01

    The U isotope system represents a relatively new paleoredox proxy that can help trace the evolution of global ocean redox chemistry, but has rarely been applied to the Mid-Proterozoic. We report U isotope data for marine black shales of the early Mesoproterozoic Velkerri Formation (Roper Group) and late Paleoproterozoic Wollogorang Formation (Tawallah Group) from the McArthur Basin, Northern Australia. An average authigenic δ238U of 0.13 ± 0.04‰ (1SD; relative to standard CRM145) was obtained for six euxinic shales from a 1 m interval that previously yielded a precise Re-Os depositional age of 1361 ± 21 Ma. After correcting for a U isotope fractionation of 0.60-0.85‰ between seawater and open-ocean euxinic sediments, we infer that coeval global seawater had a δ238U of -0.47‰ to -0.72‰, which is 0.1-0.3‰ lighter than modern seawater (-0.40 ± 0.03‰). A U isotope mass-balance model suggests that anoxic marine environments accounted for 25-50% of the global oceanic U sink at 1.36 Ga, which is 3-7 times greater than today. The model suggests that a significant proportion, potentially even a majority, of the seafloor was not covered by anoxic waters. Hence, we infer that a significant extent of the ocean floor was covered by O2-bearing waters at 1.36 Ga. The O2 concentrations of those waters were not necessarily high, and a large expanse of weakly to mildly oxygenated deep waters is consistent with the U isotope data. Uranium isotope data from a 1 m interval in the lower Velkerri Formation, deposited at 1417 ± 29 Ma based on Re-Os geochronology, yield a greater estimate for the extent of ocean anoxia. Hence, the upper Velkerri Formation may capture a transient episode of increased ocean oxygenation. Previous Re-Os isotope data from black shales of the ca. 1.73 Ga Paleoproterozoic Wollogorang Formation yielded an erroneously young date of 1359 ± 150 Ma because hydrothermal fluids percolated through the Tawallah Group rocks at ca. 1640 Ma. Higher δ238U

  18. Impact of straw and rock-fragment mulches on soil moisture and early growth of holm oaks in a semiarid area

    M. N. Jimenez; J. R. Pinto; M. A. Ripoll; A. Sanchez-Miranda; F. B. Navarro

    2017-01-01

    Planted seedlings and saplings usually exhibit low survival and growth rates under dry Mediterranean environments, especially late-successional species such as Quercus. In this work, we studied the effects of straw and rock fragment mulches on the establishment conditions of holm oak (Quercus ilex L. subsp. ballota (Def.) Samp.) in SE Spain. Soil moisture was...

  19. Geochronology and geochemistry of early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China: Implications for the tectonic evolution of the eastern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Wang, Zhi-wei; Xu, Wen-liang; Pei, Fu-ping; Wang, Feng; Guo, Peng

    2016-09-01

    This paper presents new zircon U-Pb, Hf isotope, and whole-rock major and trace element data for early Paleozoic igneous rocks of the Lesser Xing'an Range, NE China, in order to constrain the early Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt (CAOB). Zircon U-Pb dating indicates that early Paleozoic magmatic events within the northern Songnen-Zhangguangcai Range Massif (SZM) can be subdivided into four stages: Middle Cambrian ( 505 Ma), Late Cambrian ( 490 Ma), Early-Middle Ordovician ( 470 Ma), and Late Ordovician (460-450 Ma). The Middle Cambrian monzogranites are K-rich, weakly to strongly peraluminous, and characterized by pronounced heavy rare earth element (HREE) depletions, high Sr/Y ratios, low Y concentrations, low primary zircon εHf(t) values (- 6.79 to - 1.09), and ancient two-stage model (TDM2) ages (1901-1534 Ma). These results indicate derivation from partial melting of thickened ancient crustal materials that formed during the amalgamation of the northern SZM and the northern Jiamusi Massif (JM). The Late Cambrian monzonite, quartz monzonite, and monzogranite units are chemically similar to A-type granites, and contain zircons with εHf(t) values of - 2.59 to + 1.78 and TDM2 ages of 1625-1348 Ma. We infer that these rocks formed from primary magmas generated by partial melting of Mesoproterozoic accreted lower crustal materials in a post-collisional extensional environment. The Early-Middle Ordovician quartz monzodiorite, quartz monzonite, monzogranite, and rhyolite units are calc-alkaline, relatively enriched in light REEs (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), depleted in HREEs and high field strength elements (HFSEs; e.g., Nb, Ta, and Ti), and contain zircons with εHf(t) values of - 7.33 to + 4.98, indicative of formation in an active continental margin setting. The Late Ordovician alkali-feldspar granite and rhyolite units have A-type granite affinities that suggest they formed in an

  20. The Rocks of the Columbia Hills

    NASA Technical Reports Server (NTRS)

    Squyres, Steven W.; Arvidson, Raymond E.; Blaney, Diana L.; Clark, Benton C.; Crumpler, Larry; Farrand, William H.; Gorevan, Stephen; Herkenhoff, Kenneth; Hurowitz, Joel; Kusack, Alastair; hide

    2006-01-01

    The Mars Exploration Rover Spirit has identified five distinct rock types in the Columbia Hills of Gusev crater. Clovis Class rock is a poorly-sorted clastic rock that has undergone substantial aqueous alteration. We interpret it to be aqueously-altered ejecta deposits formed by impacts into basaltic materials. Wishstone Class rock is also a poorly-sorted clastic rock that has a distinctive chemical composition that is high in Ti and P and low in Cr. Wishstone Class rock may be pyroclastic in origin. Peace Class rock is a sedimentary material composed of ultramafic sand grains cemented by significant quantities of Mg- and Ca-sulfates. Peace Class rock may have formed when water briefly saturated the ultramafic sands, and evaporated to allow precipitation of the sulfates. Watchtower Class rocks are similar chemically to Wishstone Class rocks, and have undergone widely varying degrees of near-isochemical aqueous alteration. They may also be ejecta deposits, formed by impacts into Wishstone-rich materials and altered by small amounts of water. Backstay Class rocks are basalt/trachybasalt lavas that were emplaced in the Columbia Hills after the other rock classes were, either as impact ejecta or by localized volcanic activity. The geologic record preserved in the rocks of the Columbia Hills reveals a period very early in martian history in which volcanic materials were widespread, impact was a dominant process, and water was commonly present.

  1. Geochronology, geochemistry, and petrogenesis of late Permian to early Triassic mafic rocks from Darongshan, South China: Implications for ultrahigh-temperature metamorphism and S-type granite generation

    NASA Astrophysics Data System (ADS)

    Xu, Wang-Chun; Luo, Bi-Ji; Xu, Ya-Jun; Wang, Lei; Chen, Qi

    2018-05-01

    The role of the mantle in generating ultrahigh-temperature metamorphism and peraluminous S-type granites, and the extent of crust-mantle interaction are topics fundamental to our understanding of the Earth's evolution. In this study we present geochronological, geochemical, and Sr-Nd-Hf isotopic data for dolerites and mafic volcanic rocks from the Darongshan granite complex belt in western Cathaysia, South China. LA-ICP-MS U-Pb zircon analyses yielded magma crystallization ages of ca. 250-248 Ma for the dolerites, which are coeval with eruption of the mafic volcanic rocks, ultrahigh-temperature metamorphism, and emplacement of S-type granites in the Darongshan granite complex belt. The mafic volcanic rocks are high-K calc-alkaline or shoshonitic, enriched in Th, U, and light rare earth elements, and depleted in Nb, Ta and Ti. The dolerites are characterized by high Fe2O3tot (11.61-20.39 wt%) and TiO2 (1.62-3.17 wt%), and low MgO (1.73-4.38 wt%), Cr (2.8-10.8 ppm) and Ni (2.5-11.4 ppm). Isotopically, the mafic volcanic rocks have negative whole-rock εNd(t) values (-6.7 to -9.0) and high ISr values (0.71232 to 0.71767), which are slightly depleted compared with the dolerite samples (εNd(t) = -10.3 to -10.4 and ISr = 0.71796 to 0.71923). Zircons in the dolerites have εHf(t) values of -7.6 to -10.9. The mafic volcanic rocks are interpreted to have resulted from the partial melting of an enriched lithospheric mantle source with minor crustal contamination during ascent, whereas the dolerites formed by late-stage crystallization of enriched lithospheric mantle-derived magmas after fractionation of olivine and pyroxene. The formation of these mantle-derived mafic rocks may be attributed to transtension along a NE-trending strike-slip fault zone that was related to oblique subduction of the Paleo-Pacific plate beneath South China. Such underplated mafic magmas would provide sufficient heat for the generation of ultrahigh-temperature metamorphism and S-type granites, and

  2. Geochronology and geochemistry of the Early Jurassic Yeba Formation volcanic rocks in southern Tibet: Initiation of back-arc rifting and crustal accretion in the southern Lhasa Terrane

    NASA Astrophysics Data System (ADS)

    Wei, Youqing; Zhao, Zhidan; Niu, Yaoling; Zhu, Di-Cheng; Liu, Dong; Wang, Qing; Hou, Zengqian; Mo, Xuanxue; Wei, Jiuchuan

    2017-05-01

    Understanding the geological history of the Lhasa Terrane prior to the India-Asia collision ( 55 ± 10 Ma) is essential for improved models of syn-collisional and post-collisional processes in the southern Lhasa Terrane. The Miocene ( 18-10 Ma) adakitic magmatism with economically significant porphyry-type mineralization has been interpreted as resulting from partial melting of the Jurassic juvenile crust, but how this juvenile crust was accreted remains poorly known. For this reason, we carried out a detailed study on the volcanic rocks of the Yeba Formation (YF) with the results offering insights into the ways in which the juvenile crust may be accreted in the southern Lhasa Terrane in the Jurassic. The YF volcanic rocks are compositionally bimodal, comprising basalt/basaltic andesite and dacite/rhyolite dated at 183-174 Ma. All these rocks have an arc-like signature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba and U) and light rare earth elements (LREEs) and depleted high field strength elements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr-Nd and zircon Hf isotopic compositions, pointing to significant mantle isotopic contributions. Modeling results of trace elements and isotopes are most consistent with the basalts being derived from a mantle source metasomatized by varying enrichment of subduction components. The silicic volcanic rocks show the characteristics of transitional I-S type granites, and are best interpreted as resulting from re-melting of a mixed source of juvenile amphibole-rich lower crust with reworked crustal materials resembling metagraywackes. Importantly, our results indicate northward Neo-Tethyan seafloor subduction beneath the Lhasa Terrane with the YF volcanism being caused by the initiation of back-arc rifting. The back-arc setting is a likely site for juvenile crustal accretion in the southern Lhasa Terrane.

  3. Lower-crustal xenoliths from Jurassic kimberlite diatremes, upper Michigan (USA): Evidence for Proterozoic orogenesis and plume magmatism in the lower crust of the southern Superior Province

    Zartman, Robert E.; Kempton, Pamela D.; Paces, James B.; Downes, Hilary; Williams, Ian S.; Dobosi, Gábor; Futa, Kiyoto

    2013-01-01

    Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/€204Pb vs 207Pb/€204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/€204Pb vs 206Pb/€204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a

  4. Application of Radial Basis Functional Link Networks to Exploration for Proterozoic Mineral Deposits in Central Iran

    SciT

    Behnia, Pouran

    2007-06-15

    The metallogeny of Central Iran is characterized mainly by the presence of several iron, apatite, and uranium deposits of Proterozoic age. Radial Basis Function Link Networks (RBFLN) were used as a data-driven method for GIS-based predictive mapping of Proterozoic mineralization in this area. To generate the input data for RBFLN, the evidential maps comprising stratigraphic, structural, geophysical, and geochemical data were used. Fifty-eight deposits and 58 'nondeposits' were used to train the network. The operations for the application of neural networks employed in this study involve both multiclass and binary representation of evidential maps. Running RBFLN on different input datamore » showed that an increase in the number of evidential maps and classes leads to a larger classification sum of squared error (SSE). As a whole, an increase in the number of iterations resulted in the improvement of training SSE. The results of applying RBFLN showed that a successful classification depends on the existence of spatially well distributed deposits and nondeposits throughout the study area.« less

  5. Biogeochemical Cycling of Methane in the Proterozoic and Its Role in the Carbon Isotope Budget

    NASA Astrophysics Data System (ADS)

    Schrag, D. P.; Laakso, T.

    2016-12-01

    Various studies have proposed that the biogeochemical cycle of methane has played an important role throughout Earth history, both in contributing to greenhouse stability of climate in the Archean and producing carbon isotope variations and climate fluctuations in the Proterozoic and Phanerozoic. Using a simple box model that couples the geochemical cycles on carbon, oxygen, hydrogen, iron, and sulfur, combined with recent studies of methane cycling in anoxic environments, we reexamine the role of methane in both the Archean and Proterozoic, focusing on methane's role in the carbon isotope budget. We find that methane plays a much more modest role at all times of relative anoxia in the deep ocean, which requires an alternative explanation for the carbon isotope record, in particular the "boring billion" during the Mesoproterozoic. In particular, the high burial efficiency driven by lower oxygen levels drives primary production to much lower levels than has been previously described, resulting in relatively little organic matter available for methanogenesis. In addition, the anoxia in deep water results in a reduced role for methanotrophy at these times, and therefore a change in the mechanisms for production of authigenic carbonate, which may have played a significant role in the carbon isotope budget.

  6. The Sr isotope chemostratigraphy as a tool for solving stratigraphic problems of the Upper Proterozoic (Riphean and Vendian)

    NASA Astrophysics Data System (ADS)

    Kuznetsov, A. B.; Semikhatov, M. A.; Gorokhov, I. M.

    2014-11-01

    Published and original data on the Sr isotopic characterization of carbonates from the Riphean and Vendian key sections of the Southern Urals, Siberia, Asia, Africa, Australia, and North America are considered in compliance with the suggested principles of reconstructing the Sr isotopic composition of the Proterozoic seawater. The suggested methodic approach is used to plot the reference curve of the 87Sr/86Sr variations in the Riphean and Vendian oceans. During the time span of 1600-1250 Ma, the 87Sr/86Sr variations were in a narrow range corresponding to 0.70456-0.70494, but approaching the date of about 1030 Ma, the 87Sr/86Sr ratio rose to 0.70601-0.70611 and then quickly declined to 0.70519-0.70523 near the date of 1000 Ma. In the second half of the late Riphean and in the Vendian, the ratio grew almost steadily from 0.70521-0.70535 to values of 0.70874-0.70885 characteristic of the Late Vendian time. The subsequent regular growth of that ratio in seawater lasted from 840 to 550 Ma, though there were short-term epochs when the ratio noticeably dropped to 0.70561-0.70575 at approximately 760 Ma and to 0.70533-0.70538 at 670-660 Ma. After the mid-Late Vendian maximum, it declined to 0.70812-0.70823 at the end of the Nemakit-Daldynian Age and decreased to 0.70806-0.70812 during the Tommotian Age of the Early Cambrian. As is shown, the Sr isotopic variations in the Riphean and Vendian oceans were interrelated with global tectonic events in geospheres and formation stages of the Rodinia and Gondwana supercontinents. The Baikalian Complex of Siberia is considered in the work as a case in point illustrating advantages of the expounded approach with respect to age substantiation of particular stratigraphic subdivisions.

  7. Thermal Inertia of Rocks and Rock Populations

    NASA Technical Reports Server (NTRS)

    Golombek, M. P.; Jakosky, B. M.; Mellon, M. T.

    2001-01-01

    The effective thermal inertia of rock populations on Mars and Earth is derived from a model of effective inertia versus rock diameter. Results allow a parameterization of the effective rock inertia versus rock abundance and bulk and fine component inertia. Additional information is contained in the original extended abstract.

  8. Raster Images of Geologic Maps of Middle Proterozoic Belt strata in parts of Benewah, Bonner, Kootenai and Shoshone Counties, Idaho and Lincoln, Mineral and Sanders Counties, Montana

    Boleneus, David E.; Appelgate, Larry M.; Joseph, Nancy L.; Brandt, Theodore R.

    2001-01-01

    Geologic maps of the western part of the Belt Basin of western Montana and northern Idaho were converted into digital raster (TIFF image) format to facilitate their manipulation in geographic information systems. The 85-mile x 100-mile map area mostly contains rocks belonging to the lower and middle Belt Supergroup. The area is of interest as these Middle Proterozoic strata contain vein-type lead-zinc-silver deposits in the Coeur d?Alene Mining District in the St. Regis and Revett formations and strata-bound copper-silver deposits, such as the Troy mine, within the Revett Formation. The Prichard Formation is also prospective for strata-bound lead-zinc deposits because equivalent Belt strata in southern British Columbia, Canada host the Sullivan lead-zinc deposit. Map data converted to digital images include 13 geological maps at scales ranging from 1:48,000 to 1:12,000. Geologic map images produced from these maps by color scanning were registered to grid tick coverages in a Universal Transverse Mercator (North American Datum of 1927, zone 11) projection using ArcView Image Analysis. Geo-registering errors vary from 10 ft to 114 ft.

  9. The furnace in the basement: Part 1, The early days of the Hot Dry Rock Geothermal Energy Program, 1970--1973

    SciT

    Smith, M.C.

    1995-09-01

    This report presents the descriptions of the background information and formation of the Los Alamos Scientific Laboratory Geothermal Energy Group. It discusses the organizational, financial, political, public-relations,geologic, hydrologic, physical, and mechanical problems encountered by the group during the period 1970--1973. It reports the failures as well as the successes of this essential first stage in the development of hot dry rock geothermal energy systems.

  10. Rock Driller

    NASA Technical Reports Server (NTRS)

    Peterson, Thomas M.

    2001-01-01

    The next series of planetary exploration missions require a method of extracting rock and soil core samples. Therefore a prototype ultrasonic core driller (UTCD) was developed to meet the constraints of Small Bodies Exploration and Mars Sample Return Missions. The constraints in the design are size, weight, power, and axial loading. The ultrasonic transducer requires a relatively low axial load, which is one of the reasons this technology was chosen. The ultrasonic generator breadboard section can be contained within the 5x5x3 limits and weighs less than two pounds. Based on results attained the objectives for the first phase were achieved. A number of transducer probes were made and tested. One version only drills, and the other will actually provide a small core from a rock. Because of a more efficient transducer/probe, it will run at very low power (less than 5 Watts) and still drill/core. The prototype generator was built to allow for variation of all the performance-effecting elements of the transducer/probe/end effector, i.e., pulse, duty cycle, frequency, etc. The heart of the circuitry is what will be converted to a surface mounted board for the next phase, after all the parameters have been optimized and the microprocessor feedback can be installed.

  11. Zircon-pyrochlore ores of Proterozoic Gremyakha-Vyrmes polyphase massif, Kola Peninsula: source and evolution

    NASA Astrophysics Data System (ADS)

    Sorokhtina, Natalia; Belyatsky, Boris; Antonov, Anton; Kononkova, Natalia; Lepekhina, Elena; Kogarko, Lia

    2017-04-01

    The alkaline-ultrabasic Gremyakha-Vyrmes massif occurs within the Central Kola terrane in the northern part of the Fennoscandian Shield and consists of diverse rock complexes: basic-ultrabasic rocks, foidolites, alkaline metasomatic rocks and carbonatites, alkaline granites and granosyenites. Nb-Zr ore deposit is confined to alkaline metasomatic rocks developed over foidolites. The metasomatites are represented by albitites and aegirinites occur as submeridionally orientated zones extending up to 6-8 km and several hundred meters thickness. They are mainly composed of albite and aegirine, but amphibole, annite, microcline, fluorapatite, titanite, ilmenite, pyrochlore group minerals, zircon are present [Sorokhtina et al., 2016]. Carbonatites are developed sporadically and accessory zircon but not the pyrochlore is observed only in contact zones with albitites and aegerinites. In metasomatites, zircon and pyrochlore are main rare metal minerals, which are formed at the latest stages of crystallization. Ca-dominant fluorcalcio- and hydroxycalciopyrochlores are the most abundant, whereas U-dominant pyrochlore, oxyuranobetafite, zero-valent-dominant (Ba, Sr-dominant) pyrochlore, hydro- or kenopyrochlore are rare. The pyrochlore-group minerals form heterogeneous metacrystals containing inclusions of host rock minerals, calcite, ilmenite, zircon, sulfides, and graphite. While pyrochlore is replaced by Si-rich "pyrochlore" (SiO2 is up to 18 wt.%.), cation-deficient hydrated pyrochlore, Fe-Si-Nb, U-Si-Nb, and Al-Si-Nb phases along fracture zones and margins. The early generation zircon is represented by large heterogeneous metacrystals filled with inclusions of various host rock minerals, calcite, ilmenite, thorite, thorianite and sulfides, while the late zircons are empty of inclusions. Zircons are nearly stoichiometric in composition; but intermediate zones are enriched in Pb, Y and Th, and overgrowths are enriched Hf only. According to CL and ion-microprobe analysis

  12. Tectonic and regional metamorphic implications of the discovery of Middle Ordovician conodonts in cover rocks east of the Green Mountain massif, Vermont

    Ratcliffe, N.M.; Harris, A.G.; Walsh, G.J.

    1999-01-01

    Middle Ordovician (late Arenigian - early Caradocian) conodonts were recovered from a dolostone lens in carbonaceous schist 30 m below the base of the Pinney Hollow Formation in the Eastern Cover sequence near West Bridgewater, Vermont. These are the first reported fossils from the metamorphic cover sequence rocks east of the Green Mountain, Berkshire, and Housatonic massifs of western New England. The conodonts are recrystallized, coated with graphitic matter, thermally altered to a color alteration index (CAI) of at least 5, and tectonically deformed. The faunule is nearly monospecific, consisting of abundant Periodon aculeatus Hadding? and rare Protopanderodus. The preponderance of Periodon and the absence of warm, shallow-water species characteristic of the North American Midcontinent Conodont Province suggest a slope or basin depositional setting. The conodont-bearing carbonaceous schist is traceable 3 km southeast to the Plymouth area, where it had been designated the uppermost member of the Plymouth Formation, previously regarded as Early Cambrian in age. The age and structural position of the carbonaceous schist above dolostones of the Plymouth Formation but below the Pinney Hollow Formation (upper Proterozoic and Lower Cambrian?) suggest that this unit may be correlative or time transgressive with the Ira Formation, which underlies the Taconic allochthons in the Vermont Valley. Such a correlation supports the concept of placing the western limit of the root zone of the Taconic allochthons beneath the Pinney Hollow Formation. An approximate absolute age assignment for the conodont-bearing rock is between 470 and 454 Ma. This suggests that dynamothermal metamorphism during the Taconian orogeny on the east flank of the Green Mountains was younger than early Caradocian, which is in accord with the middle Caradocian age of the Ira Formation west of the Green Mountain massif.

  13. Reactivation of the Archean-Proterozoic suture along the southern margin of Laurentia during the Mazatzal orogeny: Petrogenesis and tectonic implications of ca. 1.63 Ga granite in southeastern Wyoming

    Jones, Daniel S.; Barnes, Calvin G.; Premo, Wayne R.; Snoke, Arthur W.

    2013-01-01

    The presence of ca. 1.63 Ga monzogranite (the “white quartz monzonite”) in the southern Sierra Madre, southeastern Wyoming, is anomalous given its distance from the nearest documented plutons of similar age (central Colorado) and the nearest contemporaneous tectonic margin (New Mexico). It is located immediately south of the Cheyenne belt—a ca. 1.75 Ga Archean-Proterozoic tectonic suture. New geochronological, isotopic, and geochemical data suggest that emplacement of the white quartz monzonite occurred between ca. 1645 and 1628 Ma (main pulse ca. 1628 Ma) and that the white quartz monzonite originated primarily by partial melting of the Big Creek Gneiss, a modified arc complex. There is no evidence that mafic magmas were involved. Open folds of the ca. 1750 Ma regional foliation are cut by undeformed white quartz monzonite. On a regional scale, rocks intruded by the white quartz monzonite have experienced higher pressure and temperature conditions and are migmatitic as compared to the surrounding rocks, suggesting a genetic relationship between the white quartz monzonite and tectonic exhumation. We propose that regional shortening imbricated the Big Creek Gneiss, uplifting the now-exposed high-grade rocks of the Big Creek Gneiss (hanging wall of the thrust and wall rock to the white quartz monzonite) and burying correlative rocks, which partially melted to form the white quartz monzonite. This tectonism is attributed to the ca. 1.65 Ga Mazatzal orogeny, as foreland shortening spread progressively into the Yavapai Province. Mazatzal foreland effects have also been described in the Great Lakes region and have been inferred in the Black Hills of South Dakota. We suggest that the crustal-scale rheologic contrast across the Archean-Proterozoic suture, originally developed along the southern margin of Laurentia, and including the Cheyenne belt, facilitated widespread reactivation of that boundary during the Mazatzal orogeny. This finding emphasizes the degree to

  14. Source rock potential in Pakistan

    SciT

    Raza, H.A.

    1991-03-01

    Pakistan contains two sedimentary basins: Indus in the east and Balochistan in the west. The Indus basin has received sediments from precambrian until Recent, albeit with breaks. It has been producing hydrocarbons since 1914 from three main producing regions, namely, the Potwar, Sulaisman, and Kirthar. In the Potwar, oil has been discovered in Cambrian, Permian, Jurassic, and Tertiary rocks. Potential source rocks are identified in Infra-Cambrian, Permian, Paleocene, and Eocene successions, but Paleocene/Eocene Patala Formation seems to be the main source of most of the oil. In the Sulaiman, gas has been found in Cretaceous and Tertiary; condensate in Cretaceousmore » rocks. Potential source rocks are indicated in Cretaceous, Paleocene, and Eocene successions. The Sembar Formation of Early Cretaceous age appears to be the source of gas. In the Kirthar, oil and gas have been discovered in Cretaceous and gas has been discovered in paleocene and Eocene rocks. Potential source rocks are identified in Kirthar and Ghazij formations of Eocene age in the western part. However, in the easter oil- and gas-producing Badin platform area, Union Texas has recognized the Sembar Formation of Early Cretaceous age as the only source of Cretaceous oil and gas. The Balochistan basin is part of an Early Tertiary arc-trench system. The basin is inadequately explored, and there is no oil or gas discovery so far. However, potential source rocks have been identified in Eocene, Oligocene, Miocene, and Pliocene successions based on geochemical analysis of surface samples. Mud volcanoes are present.« less

  15. Geophysical and petrological modeling of the lower crust and uppermost mantle in the Variscan and Proterozoic surroundings of the Trans-European Suture Zone in Central Europe

    NASA Astrophysics Data System (ADS)

    Puziewicz, Jacek; Polkowski, Marcin; Grad, Marek

    2017-04-01

    High-quality seismic data on the lower crust and uppermost lithospheric mantle in the Central European part of the Trans European Suture Zone, together with thermal and gravimetric data, enables the quantitative modeling of the rocks occurring in those parts of the lithosphere, including their mineral compositions and the chemical composition of individual minerals. The P3 seismic profile is located at the SW margin of the East European Craton. The lower crust is dominated by gabbronoritic intrusions (plagioclase An45Ab55, clinopyroxene Di80Hed20, orthopyroxene En74Fs26), and the uppermost mantle is harzburgitic (olivine and orthopyroxene Mg# 0.91). The lower crust and upper mantle of the P1 seismic profile belong to the Trans European Suture Zone, albeit the upper crust is of Variscan affinity. The P1 lower crust has gabbronoritic composition which is layered from plagioclase-rich compositions on the top to the orthopyroxene-rich ones at the bottom (plagioclase An45Ab55, clinopyroxene Di80Hed20, orthopyroxene En85Fs15), and is lithologically different Proterozoic and Variscan surroundings. The 100 × 200 km eclogite slice (garnet Alm48Gr25Py27, clinopyroxene Di51Hed10Jd39), with a thickness of 5-10 km, occurs in the uppermost mantle sampled by the P1 profile. The Niedźwiedź Massif is located at the NE margin of the Bohemian Massif, which shows an exposed Variscan basement. The lower crust beneath the Niedźwiedź Massif consists of gabbroic rock of variable proportions of plagioclase (An45Ab55) and clinopyroxene (Di80Hed20), whereas the uppermost mantle is supposedly spinel harzburgite (olivine, ortho- and clinopyroxene Mg# 0.90). Our models show that the lowermost crust and uppermost mantle of the East European Craton do not continue to the SW into the Trans European Suture Zone in its Central European section in Poland.

  16. A model for the oceanic mass balance of rhenium and implications for the extent of Proterozoic ocean anoxia

    NASA Astrophysics Data System (ADS)

    Sheen, Alex I.; Kendall, Brian; Reinhard, Christopher T.; Creaser, Robert A.; Lyons, Timothy W.; Bekker, Andrey; Poulton, Simon W.; Anbar, Ariel D.

    2018-04-01

    Emerging geochemical evidence suggests that the atmosphere-ocean system underwent a significant decrease in O2 content following the Great Oxidation Event (GOE), leading to a mid-Proterozoic ocean (ca. 2.0-0.8 Ga) with oxygenated surface waters and predominantly anoxic deep waters. The extent of mid-Proterozoic seafloor anoxia has been recently estimated using mass-balance models based on molybdenum (Mo), uranium (U), and chromium (Cr) enrichments in organic-rich mudrocks (ORM). Here, we use a temporal compilation of concentrations for the redox-sensitive trace metal rhenium (Re) in ORM to provide an independent constraint on the global extent of mid-Proterozoic ocean anoxia and as a tool for more generally exploring how the marine geochemical cycle of Re has changed through time. The compilation reveals that mid-Proterozoic ORM are dominated by low Re concentrations that overall are only mildly higher than those of Archean ORM and significantly lower than many ORM deposited during the ca. 2.22-2.06 Ga Lomagundi Event and during the Phanerozoic Eon. These temporal trends are consistent with a decrease in the oceanic Re inventory in response to an expansion of anoxia after an interval of increased oxygenation during the Lomagundi Event. Mass-balance modeling of the marine Re geochemical cycle indicates that the mid-Proterozoic ORM with low Re enrichments are consistent with extensive seafloor anoxia. Beyond this agreement, these new data bring added value because Re, like the other metals, responds generally to low-oxygen conditions but has its own distinct sensitivity to the varying environmental controls. Thus, we can broaden our capacity to infer nuanced spatiotemporal patterns in ancient redox landscapes. For example, despite the still small number of data, some mid-Proterozoic ORM units have higher Re enrichments that may reflect a larger oceanic Re inventory during transient episodes of ocean oxygenation. An improved understanding of the modern oceanic Re

  17. Isotopic complexities and the age of the Delfonte volcanic rocks, eastern Mescal Range, southeastern California: Stratigraphic and tectonic implications

    Fleck, R.J.; Mattinson, J.M.; Busby, C.J.; Carr, M.D.; Davis, G.A.; Burchfiel, B.C.

    1994-01-01

    Combined U-Pb zircon, Rb-Sr, 40Ar/39Ar laser-fusion, and conventional K-Ar geochronology establish a late Early Cretaceous age for the Delfonte volcanic rocks. U-Pb zircon analyses define a lower intercept age of 100.5 ± 2 Ma that is interpreted as the crystallization age of the Delfonte sequence. Argon studies document both xenocrystic contamination and postemplacement Ar loss. Rb-Sr results from mafic lavas at the base of the sequence demonstrate compositionally correlated variations in initial 87Sr/86Sr ratios (Sri) from 0.706 for basalts to 0.716 for andesitic compositions. This covariation indicates substantial mixing of subcontinental lithosphere with Proterozoic upper crust. Correlations between Rb/Sr and Sri may result not only in pseudoisochrons approaching the age of the crustal component, but also in reasonable but incorrect apparent ages approaching the true age.Ages obtained in this study require that at least some of the thrust faulting in the Mescal Range-Clark Mountain portion of the foreland fold-and-thrust belt occurred later than ca. 100 Ma and was broadly contemporaneous with emplacement of the Keystone thrust plate in the Spring Mountains to the northeast. Comparison of the age and Rb-Sr systematics of ash-flow tuff boulders in the synorogenic Lavinia Wash sequence near Goodsprings, Nevada, with those of the Delfonte volcanic rocks supports a Delfonte source for the boulders. The 99 Ma age of the Lavinia Wash sequence is nearly identical to the Delfonte age, requiring rapid erosion, transport, and deposition following Delfonte volcanism.

  18. Did in-place rotation of South America during the Early Cretaceous create both the early South Atlantic rift/salt basin and the Paraná-Etendeka large igneous province? Peter Szatmari1 and Edison J. Milani1 1Petrobras Research Center (CENPES) Geological Research & Development (PDGEO), Ilha do Fundão, Rio de Janeiro, Brazil

    NASA Astrophysics Data System (ADS)

    Szatmari, P.; Milani, E.

    2012-12-01

    Large igneous provinces with continental flood basalts, some related to rifting, have been traditionally attributed to mantle plume heads rising from the lower mantle. The early Cretaceous South Atlantic rift, an archetype of plate tectonics, and the Paraná-Etendeka continental flood basalts on land outside the rift, formed as South America rotated clockwise about a pole in its northeastern tip (Rabinowitz & LaBrecque, 1979), away from Africa and toward the subduction zone on its Pacific margin. This rotation opened the early South Atlantic southward while it kept the Equatorial Atlantic gateway to the Central Atlantic and the Tethys closed by compression. Rifting started in the late Jurassic in the extreme south, near the subduction zone at the continent's southern tip. It rapidly propagated NNE, mainly along inherited late Proterozoic (mostly Ediacaran) fold belts, and reached what has later become the eastern end of the Equatorial margin still in latest Jurassic time. Massive mostly basaltic volcanism peaked about 20 Ma later in Hauterivian time (136 to 130 Ma), forming dike swarms which, in the south, are accompanied by flood basalts of the Paraná-Etendeka large igneous province. The massive rise of mostly tholeiitic magma resulted from hotspot-like high temperatures prevailing beneath the cold and thick Gondwana lithosphere that had remained unbroken since Proterozoic times for about 400 Ma. Early basalt dike swarms trending E-W and SE-NW were transversal to the rift. They are two-three hundred kilometers long and 1000-2000 km apart, penetrating far into the continent's unrifted lithosphere and cutting through all inherited Proterozoic structures that controlled rifting. The successive basalt dike swarms (and their individual dikes) increase in thickness to the southwest, away from the continent's pole of rotation, as does the width of the rift. The E-W-trending Ceará-Mirim dike swarm occurs in the extreme northeast of the continent. Further southwest the

  19. LA-ICP-MS zircon U-Pb and muscovite K-Ar ages of basement rocks from the south arm of Sulawesi, Indonesia

    NASA Astrophysics Data System (ADS)

    Jaya, Asri; Nishikawa, Osamu; Hayasaka, Yasutaka

    2017-11-01

    The zircon U-Pb and muscovite K-Ar age from the Bantimala, Barru and Biru basement complexes in the South Arm of Sulawesi, Indonesia provide new information regarding the timing of magmatism, metamorphism and sedimentation in this region and have implications for the origin and evolution of the study area. The study area is at the juncture between the southeast margin of Sundaland and Bird's Head-Australia. The age of both the zircon U-Pb of detrital materials in the Bantimala Complex and the muscovite K-Ar of amphibolite in the Biru Complex fall in the Late Early Cretaceous (between 109 and 115 Ma), which is a similar age range to previous data for both the sedimentary and metamorphic rocks. The youngest detrital zircon in the schist samples from the Barru Complex fall into the Triassic in age (between 243 and 247 Ma). These age data indicate that the protolith of all three basement complexes were involved in the subduction system and metamorphosed in the late Early Cretaceous, but there are several differences in their deposition environment under and out of the influence of the late Early Cretaceous magmatism in the Bantimala and Barru Complexes, respectively. Felsic igneous activities are confirmed in the Late Cretaceous and the Eocene by the zircon U-Pb age of igneous rocks intruding or included as detrital fragments in three basement complexes. These dates are similar to those reported from the Meratus Complex of South Kalimantan. The detrital zircon age distributions of the basement rocks in the South Arm of Sulawesi display predominant Mesozoic (Cretaceous and Triassic) and Paleozoic populations with a small population of Proterozoic ages supporting the hypothesis that the West Sulawesi block originated from the region of the circum Bird's Head-Australian, namely the Inner Banda block. The absence of Jurassic zircon age population in the South Arm of Sulawesi suggests the division of the South Arm of Sulawesi from the Inner Banda block in early stage of

  20. Odyssey/White Rock

    NASA Technical Reports Server (NTRS)

    2002-01-01

    These Mars Odyssey images show the 'White Rock' feature on Mars in both infrared (left) and visible (right) wavelengths. The images were acquired simultaneously on March 11, 2002. The box shows where the visible image is located in the infrared image. 'White Rock' is the unofficial name for this unusual landform that was first observed during the Mariner 9 mission in the early 1970's. The variations in brightness in the infrared image are due to differences in surface temperature, where dark is cool and bright is warm. The dramatic differences between the infrared and visible views of White Rock are the result of solar heating. The relatively bright surfaces observed at visible wavelengths reflect more solar energy than the darker surfaces, allowing them to stay cooler and thus they appear dark in the infrared image. The new thermal emission imaging system data will help to address the long standing question of whether the White Rock deposit was produced in an ancient crater lake or by dry processes of volcanic or wind deposition. The infrared image has a resolution of 100 meters (328 feet) per pixel and is 32 kilometers (20 miles) wide. The visible image has a resolution of 18 meters per pixel and is approximately 18 kilometers (11 miles) wide. The images are centered at 8.2 degrees south latitude and 24.9 degrees east longitude.

    NASA's Jet Propulsion Laboratory manages the 2001 Mars Odyssey mission for NASA's Office of Space Science, Washington, D.C. The Thermal Emission Imaging System was developed by Arizona State University, Tempe, in collaboration with Raytheon Santa Barbara Remote Sensing. Lockheed Martin Astronautics, Denver, is the prime contractor for the Odyssey project, and developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL, a division of the California Institute of Technology in Pasadena.

  1. Late proterozoic and paleozoic tides, retreat of the moon, and rotation of the earth

    Sonett, C.P.; Kvale, E.P.; Zakharian, A.; Chan, M.A.; Demko, T.M.

    1996-01-01

    The tidal rhythmites in the Proterozoic Big Cottonwood Formation (Utah, United States), the Neoproterozoic Elatina Formation of the Flinders Range (southern Australia), and the Lower Pennsylvanian Pottsville Formation (Alabama, United States) and Mansfield Formation (Indiana, United States) indicate that the rate of retreat of the lunar orbit is d??/dt k2 sin(2??) (where ?? is the Earth-moon radius vector, k2 is the tidal Love number, and ?? is the tidal lag angle) and that this rate has been approximately constant since the late Precambrian. When the contribution to tidal friction from the sun is taken into account, these data imply that the length of the terrestrial day 900 million years ago was -18 hours.

  2. Fractal branching organizations of Ediacaran rangeomorph fronds reveal a lost Proterozoic body plan.

    PubMed

    Hoyal Cuthill, Jennifer F; Conway Morris, Simon

    2014-09-09

    The branching morphology of Ediacaran rangeomorph fronds has no exact counterpart in other complex macroorganisms. As such, these fossils pose major questions as to growth patterns, functional morphology, modes of feeding, and adaptive optimality. Here, using parametric Lindenmayer systems, a formal model of rangeomorph morphologies reveals a fractal body plan characterized by self-similar, axial, apical, alternate branching. Consequent morphological reconstruction for 11 taxa demonstrates an adaptive radiation based on 3D space-filling strategies. The fractal body plan of rangeomorphs is shown to maximize surface area, consistent with diffusive nutrient uptake from the water column (osmotrophy). The enigmas of rangeomorph morphology, evolution, and extinction are resolved by the realization that they were adaptively optimized for unique ecological and geochemical conditions in the late Proterozoic. Changes in ocean conditions associated with the Cambrian explosion sealed their fate.

  3. Sr isotopic variations in Upper Proterozoic carbonates from Svalbard and East Greenland

    SciT

    Derry, L.A.; Keto, L.S.; Jacobsen, S.B.

    1989-09-01

    The authors report initial {sup 87}Sr/{sup 86}Sr values from an Upper Proterozoic carbonate succession from Svalbard and East Greenland. This succession, now tectonically separated into three sequences, is thick, relatively continuous, and well preserved. The relative ages of the samples from within the basin are well constrained by litho-, bio-, and chemostratigraphic techniques. The data from this study and related data from the literature are used to construct a curve of {sup 87}Sr/{sup 86}Sr for Upper Proterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period between 650 andmore » 800 Ma. The data indicate that {Delta}{sup 87}Sr values of seawater were variable but low ({Delta}{sup 87}Sr {approximately}{minus}500 to {minus}250) between 900 and 650 Ma, and rose rapidly to {approximately} +30 by 600 Ma. The range of variation of {Delta}{sup 87}Sr in seawater during the Riphean-Vendian exceeds the entire range of {Delta}{sup 87}Sr in seawater during the Phanerozoic. While variation in the average isotopic composition of Sr delivered to the oceans by rivers can account for some of the observed range, changes in the ratio of submarine hydrothermal flux to river water (continental) flux are responsible for the large variation in seawater Sr isotopic composition. Changes in the continental flux of Sr to the oceans can be related to tectonic factors. Large changes in the hydrothermal flux to river water flux ratio indicated by the data could have significant consequences for the chemistry of the ocean-atmosphere system.« less

  4. Sr isotopic variations in Upper Proterozoic carbonates from Svalbard and East Greenland.

    PubMed

    Derry, L A; Keto, L S; Jacobsen, S B; Knoll, A H; Swett, K

    1989-01-01

    We report initial 87Sr/86Sr values from an Upper Proterozoic carbonate succession from Svalbard and East Greenland. This succession, now tectonically separated into three sequences, is thick, relatively continuous, and well preserved. The relative ages of the samples from within the basin are well constrained by litho-, bio-, and chemostratigraphic techniques. The data from this study and related data from the literature are used to construct a curve of 87Sr/86Sr for Upper Proterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period between 650 and 800 Ma. The data indicate that delta 87Sr values of seawater were variable but low (delta 87Sr approximately -500 to -250) between 900 and 650 Ma, and rose rapidly to approximately +30 by 600 Ma. The range of variation of delta 87Sr in seawater during the Riphean-Vendian exceeds the entire range of delta 87Sr in seawater during the Phanerozoic. While variation in the average isotopic composition of Sr delivered to the oceans by rivers can account for some of the observed range, changes in the ratio of submarine hydrothermal flux to river water (continental) flux are responsible for the large variation in seawater Sr isotopic composition. Changes in the continental flux of Sr to the oceans can be related to tectonic factors. Large changes in the hydrothermal flux to river water flux ratio indicated by the data could have significant consequences for the chemistry of the ocean-atmosphere system.

  5. Stratigraphy of the Proterozoic Revett Formation, Coeur d'Alene District, Idaho

    Mauk, Jeffrey L.

    2002-01-01

    The Proterozoic Revett Formation of the Belt Supergroup contains three informal members that can be identified throughout the Coeur d'Alene mining district of northern Idaho. The lower Revett Formation is dominated by quartzite, but also contains intervals of siltite. The middle Revett consists predominantly of siltite, though quartzite and argillite locally form significant intervals. The upper Revett consists of intervals of quartzite that alternate with intervals of siltite and/or thin-bedded argillite. These units show dramatic changes in thickness and sedimentary facies within the Coeur d'Alene mining district; changes that are more abrupt and extreme than seen elsewhere in the Belt basin. The regionally significant Osburn fault bisects the district, with 20 to 30 km of post-mineralization right-lateral strike-slip offset. South of this fault, the upper Revett is 640 m thick at the Bunker Hill mine in the west, 450 to 500 m thick in the centrally located Silver Belt, and over 550 m thick at the Reindeer Queen deposit to the east. North of the Osburn fault, the upper Revett is approximately 120 m thick in the vicinity of the Lucky Friday mine, but abruptly thins to 45 to 90 m to the north and northeast, in the southern end of the western Montana copper sulfide belt. The middle Revett Formation south of the Osburn fault appears to be 400 to 450 m thick. North of the Osburn Fault, the middle Revett thins to approximately 120 m in the Lucky Friday area, and to approximately 60 m at Military Gulch. The lower Revett Formation is approximately 1650 m thick south of the Osburn fault, but thins to 400 to 450 m thick to the north of the Osburn fault. Observed thickness changes support previous hypotheses that the current Osburn fault coincides with a Proterozoic synsedimentary fault that controlled sedimentation in this region.

  6. Empirical Records of Environmental Change across the Archean-Proterozoic Transition

    NASA Astrophysics Data System (ADS)

    Kaufman, A. J.

    2011-12-01

    Time-series geochemical analyses of scientific drill cores intersecting the Archean-Proterozoic transition suggest a coupling of environmental and biological change that culminated in the pervasive oxygenation of Earth's atmosphere and oceans. Elemental and multiple isotope measurements of sedimentary archives, including carbonate, shale, and banded iron-formation from Western Australia, South Africa, Brazil, and southern Canada, indicate important changes in the carbon, sulfur, and nitrogen cycles that monitor the redox state of the oceans and the cyanobacterial buildup of atmospheric oxygen and ozone. In response, continental weathering would have increased, resulting in the enhanced delivery of sulfate and nutrients to seawater, further stimulating photoautotrophic fluxes of oxygen to surface environments. The positive feedback may additionally be responsible for the decline of atmospheric methane and surface refrigeration, represented by a series of discrete ice ages beginning around 2.4 billion years ago, due to the loss of greenhouse capacity during a time of lower solar luminosity. While speculative, the linkage of surface oxidation with enhanced nutrient supply and development of stratospheric sunscreen soon after the Archean-Proterozoic boundary suggests that the earliest perturbation in the carbon cycle may be associated with the rapid expansion of single-celled eukaryotes. Both sterol synthesis in eukaryotes and aerobic respiration require significant levels of oxygen in the ambient environment. Hence, Earth's earliest ice age(s) and onset of a modern and far more energetic carbon cycle may have been directly related to the global expansion of cyanobacteria that released oxygen to the environment, and of eukaryotes that respired it.

  7. Subduction of Proterozoic to Late Triassic continental basement in the Guatemala suture zone: A petrological and geochronological study of high-pressure metagranitoids from the Chuacús complex

    NASA Astrophysics Data System (ADS)

    Maldonado, Roberto; Ortega-Gutiérrez, Fernando; Ortíz-Joya, Guillermo A.

    2018-05-01

    Many continental subduction complexes contain abundant granitic rocks coexisting with minor volumes of eclogite-facies rocks. Characterization of granitic protoliths is crucial to decipher the origin of subducted continental crust, whereas knowledge of its metamorphic evolution is required to constrain the mechanisms of burial and exhumation. In this work we present geochronological and petrological evidence that demonstrate the occurrence of a subducted Proterozoic to Late Triassic granitic basement in the Chuacús complex of central Guatemala. Metagranitoids exposed in this area are interlayered with eclogite and other high-pressure rocks, and their structure is considerably variable due to strain partitioning during deformation. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon data from two ferroan metagranites yield protolith crystallization ages of ca. 1.1 Ga and their trace-element abundances suggest an origin related to intraplate magmatism, while a high-silica, peraluminous metagranite is dated at 1.0 Ga and was probably originated by partial melting of a high-grade continental crust. On the other hand, two megacrystic to augen metagranitoids yield protolith crystallization ages of ca. 224 Ma, which are identical within errors to the protolith age of hosted eclogitic metabasites. Their high incompatible trace element abundances together with the observed spatial-temporal relationships with mafic protoliths suggest that Late Triassic bimodal magmatism in the Chuacús complex was probably originated in a within-plate setting. Regardless of their age or structure, the studied metagranites preserve evidences for high-pressure metamorphic equilibration, such as the occurrence of Ca-rich garnet (XCa up to 0.52) in association with phengite (Si contents of up to 3.4 pfu) and rutile. The integration of Zr-in-rutile thermometry and phengite barometry allows the peak metamorphic conditions to be constrained at 640-680 °C and 13 kbar. This

  8. Actin cable dynamics and Rho/Rock orchestrate a polarized cytoskeletal architecture in the early steps of assembling a stratified epithelium.

    PubMed

    Vaezi, Alec; Bauer, Christoph; Vasioukhin, Valeri; Fuchs, Elaine

    2002-09-01

    To enable stratification and barrier function, the epidermis must permit self-renewal while maintaining adhesive connections. By generating K14-GFP-actin mice to monitor actin dynamics in cultured primary keratinocytes, we uncovered a role for the actin cytoskeleton in establishing cellular organization. During epidermal sheet formation, a polarized network of nascent intercellular junctions and radial actin cables assemble in the apical plane of the monolayer. These actin fibers anchor to a central actin-myosin network, creating a tension-based plane of cytoskeleton across the apical surface of the sheet. Movement of the sheet surface relative to its base expands the zone of intercellular overlap, catalyzing new sites for nascent intercellular junctions. This polarized cytoskeleton is dependent upon alpha-catenin, Rho, and Rock, and its regulation may be important for wound healing and/or stratification, where coordinated tissue movements are involved.

  9. Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo

    NASA Astrophysics Data System (ADS)

    Weiss, Benjamin P.; Maloof, Adam C.; Tailby, Nicholas; Ramezani, Jahandar; Fu, Roger R.; Hanus, Veronica; Trail, Dustin; Bruce Watson, E.; Harrison, T. Mark; Bowring, Samuel A.; Kirschvink, Joseph L.; Swanson-Hysell, Nicholas L.; Coe, Robert S.

    2015-11-01

    It currently is unknown when Earth's dynamo magnetic field originated. Paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3.5 billion years ago (Ga). Detrital zircon crystals found in the Jack Hills of Western Australia are some of the very few samples known to substantially predate this time. With crystallization ages ranging from 3.0-4.38 Ga, these zircons might preserve a record of the missing first billion years of Earth's magnetic field history. However, a key unknown is the age and origin of magnetization in the Jack Hills zircons. The identification of >3.9 Ga (i.e., Hadean) field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz-rich conglomerates at 2.65-3.05 Ga. To address this issue, we have conducted paleomagnetic conglomerate, baked contact, and fold tests in combination with U-Pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks. These tests include the first conglomerate test directly on the Hadean-zircon bearing conglomerate at Erawandoo Hill. Although we observed little evidence for remagnetization by recent lightning strikes, we found that the Hadean zircon-bearing rocks and surrounding region have been pervasively remagnetized, with the final major overprinting likely due to thermal and/or aqueous effects from the emplacement of the Warakurna large igneous province at ∼1070 million years ago (Ma). Although localized regions of the Jack Hills might have escaped complete remagnetization, there currently is no robust evidence for pre-depositional (>3.0 Ga) magnetization in the Jack Hills detrital zircons.

  10. Chemistry and palynology of carbon seams and associated rocks from the Witwatersrand goldfields, South Africa

    Ebert, L.B.; Robbins, E.I.; Rose, K.D.; Kastrup, R.V.; Scanlon, J.C.; Gebhard, L.A.; Garcia, A.R.

    1990-01-01

    Carbon seams in the Witwatersrand System of South Africa host some of the richest gold concentrations in the world. A study of the microscopic characteristics in thin sections and acid residues, and of the chemical and physical nature of the carbon-bearing phases, was undertaken to gain some understanding of the biological precursors and thermal changes that have occurred since the seams were buried. The HClHF acid-resistant organic tissues in this Early Proterozoic coal are filamentous and spherical, which are typical morphologies for microorganisms. The tissues are carbonized black as would be expected for metamorphic rocks, so usual palynological techniques were of limited use. Therefore, the chemical and physical nature of the organic remains was studied by H C ratios, X-ray diffraction (XRD), 13C nuclear magnetic resonance (NMR), reductive chemistry, crosspolarization/magic angle spinning NMR (CP/MAS), and electron spin resonance (ESR). The H C ratios of the samples examined are similar to those of semi-anthracite and petroleum cokes from delayed cokers. XRD shows graphite is not present and that the gold is in elemental form, not chemically bound or intercalated between carbon planes. NMR shows that both aromatic and paraffinic carbons are present. Integration of the carbon NMR spectra suggests that 80% of the carbon is sp2-hybridized and 20% is sp3-hybridized. Reductive chemistry shows that the benzenoid entities are larger than common polynuclear aromatic hydrocarbons such as perylene and decacyclene. Dipolar dephasing CP/MAS NMR suggests the presence of two types of paraffinic carbons, a rigid methylene group and a rotating methyl group. The narrowing of the ESR linewidth between room temperature and 300??C shows that the materials examined have not previously been subjected to temperatures as high as 300??C. ?? 1990.

  11. UThPb age of Apollo 12 rock 12013

    Tatsumoto, M.

    1970-01-01

    A UThPb isotopic study of three chips from lunar rock 12013 indicates that parental material of the intrusion breccia formed quite early in the moon's history, possibly 3.9 to 4.3 by ago. The UThPb characteristics of the rock are distinctly different from those of other Apollo 12 igneous rocks and suggest a different origin. ?? 1970.

  12. The Proterozoic Mount Isa Fault Zone, northeastern Australia: is it really a ca. 1.9 Ga terrane-bounding suture?

    NASA Astrophysics Data System (ADS)

    Bierlein, Frank P.; Betts, Peter G.

    2004-09-01

    In marked contrast to Palaeoproterozoic Laurentia, the location of sutures and boundaries of discrete crustal fragments amalgamated during Palaeoproterozoic formation of the North Australian Craton remain highly speculative. Interpretations of suture locations have relied heavily on the analysis of regional geophysical datasets because of sparse exposure of rocks of the appropriate age. The Mount Isa Fault Zone has been interpreted as one such Palaeoproterozoic terrane-bounding suture. Furthermore, the coincidence of this fault zone with major shale-hosted massive sulphide Pb-Zn-Ag orebodies has led to speculations that trans-lithospheric faults may be an important ingredient for the development of this deposit type. This study has integrated geophysical and geochemical data to test the statute of the Mount Isa Fault as a terrane-bounding suture. Forward modelling of gravity data shows that basement rocks on either side of the Mount Isa Fault have similar densities. These interpretations are consistent with geochemical observations and Sm-Nd data that suggest that basement lithologies on either side of the Mount Isa Fault are geochemically and isotopically indistinguishable from each other, and that the Mount Isa Fault is unlikely to represent a suture zone that separates different Palaeoproterozoic terranes. Our data indicate that the crustal blocks on both sides of the Mount Isa Fault Zone must have been in within close proximity of each other since the Palaeoproterozoic, and that the Western Fold Belt was part of the (ancestral) North Australian Craton well before the ˜1.89-1.87 Ga Barramundi Orogeny. It appears that deep crustal variations in density may be related to the boundary between a shallowly west-dipping high-density mafic to ultramafic plate and low-density basement rocks. This interpretation in turn impacts on crustal-scale models for the development of shale-hosted massive sulphide Pb-Zn mineralisation, which do not require trans

  13. New geochronological history of the Mbuji-Mayi Supergroup (Proterozoic, DRC) through U-Pb and Sm-Nd dating

    NASA Astrophysics Data System (ADS)

    François, Camille; Baludikay, Blaise K.; Storme, Jean-Yves; Baudet, Daniel; Paquette, Jean-Louis; Fialin, Michel; Debaille, Vinciane; Javaux, Emmanuelle J.

    2016-04-01

    The Mbuji-Mayi Supergroup, DRC is located between the Archean-Paleoproterozoic Kasai Craton and the Mesoproterozoic Kibaran Belt. This sedimentary sequence, unaffected by regional metamorphism, preserves a large diversity of well-preserved acritarchs (organic-walled microfossils), evidencing the diversification of complex life (early eukaryotes) for the first time in mid-Proterozoic redox stratified oceans of Central Africa (Baludikay et al., in review). This Supergroup is composed of two distinct lithostratigraphic successions (i) BI Group: a lower siliciclastic sequence (ca. 1175 Myr to ca. 882 Myr or ca. 1050 Myr (Cahen, 1954; Holmes & Cahen, 1955; Delpomdor et al., 2013) unconformably overlying the ca. 2.82-2.56 Gyr granitoid Dibaya Complex to the North (Cahen & Snelling; recent notice on DRC geological map); and (ii) BII Group: a poorly age-constrained upper carbonate sequence with sparse shales . Basaltic lavas (including pillow lavas) overlying the Mbuji-Mayi Supergroup were dated around 950 Myr (Cahen et al., 1974; Cahen et al., 1984). To better constraint the age of this Supergroup in the Meso-Neoproterozoic limit, we combine different geochronological methods, in particular on diagenetic minerals such as monazite (Montel et al., 1996; Rasmussen & Muhling, 2007) and xenotime (McNaughton et al., 1999) but also on detrital zircons. For the BI Group, results of in situ U-Pb dating with LA-ICP-MS on monazite, xenotime and zircon (Laboratoire Magmas et Volcans, Clermont-Ferrand) provide ages between 2.9 and 1.2 Gyr for zircons and between 1.4 and 1.03 Gyr for monazites and xenotimes. New results of in situ U-Th-Pb dating of well-crystallized monazites and xenotimes with Electron MicroProbe (Camparis, UPMC, Paris), highlight that some crystals display zonations with an inherited core older than 1125 Myr and diagenetic rims around 1050-1075 Myr. This suggests that the diagenesis of BI Group is younger than 1175 Myr (Delpomdor et al., 2013) and probably around

  14. Rollerjaw Rock Crusher

    NASA Technical Reports Server (NTRS)

    Peters, Gregory; Brown, Kyle; Fuerstenau, Stephen

    2009-01-01

    The rollerjaw rock crusher melds the concepts of jaw crushing and roll crushing long employed in the mining and rock-crushing industries. Rollerjaw rock crushers have been proposed for inclusion in geological exploration missions on Mars, where they would be used to pulverize rock samples into powders in the tens of micrometer particle size range required for analysis by scientific instruments.

  15. Synthesis of proterozoic data as a prerequisite for tectonic and thermal modeling

    NASA Technical Reports Server (NTRS)

    Burke, K. C.

    1984-01-01

    Rocks of the Pongola supergroup form an elongate belt in the Archean Kaapvaal Caton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, it is suggested that the Pongola supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in Earth history reveals striking similarities. Rocks of the Ventersdorp Supergroup were deposited in a system of northeast trending grabens on the Kaapvaal Craton approximately 2.64 Ga ago. It is suggested that it was this collision which initiated the Ventersdorp rifting. The Ventersdorp rift province is related to an extension in the Kaapval Craton associated with the collision.

  16. Modern Microbial Ecosystems are a Key to Understanding Our Biosphere's Early Evolution and its Contributions To The Atmosphere and Rock Record

    NASA Technical Reports Server (NTRS)

    DesMarais, David J.; DeVincenzi, Donald L. (Technical Monitor)

    2000-01-01

    The survival of our early biosphere depended upon efficient coordination anion- diverse microbial populations. Microbial mats exhibit a 3.46-billion-year fossil record, thus they are the oldest known ecosystems. Photosynthetic microbial mats were key because, today, sunlight powers more than 99 percent of global primary productivity. Thus photosynthetic ecosystems have affected the atmosphere profoundly and have created the most pervasive, easily-detected fossils. Photosynthetic biospheres elsewhere will be most detectible via telescopes or spacecraft. As a part of the Astrobiology Institute, our Ames Microbial Ecosystems group examines the roles played by ecological processes in the early evolution of our biosphere, as recorded in geologic fossils and in the macromolecules of living cells: (1) We are defining the microbial mat microenvironment, which was an important milieu for early evolution. (2) We are comparing mats in contrasting environments to discern strategies of adaptation and diversification, traits that were key for long-term survival. (3) We have selected sites that mimic key environmental attributes of early Earth and thereby focus upon evolutionary adaptations to long-term changes in the global environment. (4) Our studies of gas exchange contribute to better estimates of biogenic gases in Earth's early atmosphere. This group therefore directly addresses the question: How have the Earth and its biosphere influenced each other over time Our studies strengthen the systematics for interpreting the microbial fossil record and thereby enhance astrobiological studies of martian samples. Our models of biogenic gas emissions will enhance models of atmospheres that might be detected on inhabited extrasolar planets. This work therefore also addresses the question: How can other biospheres be recogniZed" Our choice of field sites helps us explore Earth's evolving early environment. For example, modern mats that occupy thermal springs and certain freshwater

  17. The carbon-isotopic composition of Proterozoic carbonates: Riphean successions from northwestern Siberia (Anabar Massif, Turukhansk Uplift)

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Kaufman, A. J.; Semikhatov, M. A.

    1995-01-01

    Thick carbonate-dominated successions in northwestern Siberia document secular variations in the C-isotopic composition of seawater through Mesoproterozoic and early Neoproterozoic (Early to early Late Riphean) time. Mesoproterozoic dolomites of the Billyakh Group, Anabar Massif, have delta 13C values that fall between 0 and -1.9 permil versus PDB, with values in the upper part of the succession (Yusmastakh Formation) consistently higher than those of the lower (Ust'-Il'ya and Kotuikan formations). Consistent with available biostratigraphic and radiometric data, delta 13C values for Billyakh carbonates compare closely with those characterizing early Mesoproterozoic carbonates (about 1600-1200 Ma) worldwide. In contrast, late Mesoproterozoic to early Neoproterozoic limestones and dolomites in the Turukhansk Uplift exhibit moderate levels of secular variation. Only the lowermost carbonates in the Turukhansk succession (Linok Formation) have delta 13C values that approximate Billyakh values. Higher in the Turukhansk succession, delta 13C values vary from -2.7 to +4.6 permil (with outliers as low as -5.0 permil interpreted as diagentically altered). Again, consistent with paleontological and radiometric data, these values compare well with isotopic values from 1200 to 850 Ma successions elsewhere. Five sections measured in different parts of the Turukhansk basin show nearly identical patterns of variation, confirming that carbonate delta 13C correlates primarily with time and not facies. The Siberian sections illustrate the potential of integrated biostratigraphic and chemostratigraphic data in the intra- and interbasinal correlation of Mesoproterozoic and early Neoproterozoic rocks.

  18. Proterozoic Eastern Sayan ophiolites (Central Asian Orogenic Belt) record subduction initiation in vicinity of continental block

    NASA Astrophysics Data System (ADS)

    Belyaev, Vasilii; Gornova, Marina; Medvedev, Alexander; Dril, Sergey; Karimov, Anas

    2017-04-01

    Geochemical study of cumulate and volcanic rocks from ˜ 1020 Ma Eastern Sayan ophiolites1 (Siberia, Russia) is used to provide a correlation between two ophiolitic belts and link them to subduction initiation setting. Studied areas include Ospin and Ilchir massifs to the East and Dunzhugur to the West of Early Precambrian Gargan block. Ophiolitic cumulates represent peridotite-pyroxenite-gabbro-norite suite with crystallization orders of Cr-Sp - Ol - Cpx - Opx - Plag, and Cr-Sp - Ol - Opx - Amph - (Cpx) - Plag. Clinopyroxene is augite-diopside with Mg# 85-95, low Al2O3 (1-2.5%) and TiO2 (0.05-0.2%). Amph is Mg-hornblende to edenite (Mg# 84-86, 5-8% Al2O3, 0.3-0.6% TiO2). Cr-Sp has Cr# 65-83 and 0.05-0.3% TiO2 in cumulates with high Opx proportion, while in Cpx-dominating pyroxenites chemistry of Cr-Sp is variable (Cr# 40-75, 0.05-0.5% TiO2). Due to alteration, Ol and Opx chemistry is available only for some samples (Ol: Mg# 88, 0.2-0.3% NiO; Opx: Mg# 89, 1.6% Al2O3). Whole-rock MgO ranges 9 to 38%. Amph-free pyroxenites and gabbro-norites show flat to slightly depleted REE pattern with negative HFSE anomalies. Amph-pyroxenites have fractionated trace-element pattern with LREE enrichment, Nb-Ti minima at slightly higher HFSE abundances. In-situ LA-ICP-MS analysis of Cpx in Amph-free pyroxenites and gabbro-norites revealed moderately depleted to flat REE and Nb-Zr-Hf-Ti depletion, with low trace element abundances (La/SmPM = 0.14-0.9, Zr 0.6-2.3, Nd = 0.2-1.1, Yb = 0.2-0.7 ppm). Melts calculated to be in equilibrium with Cpx using distribution coefficients2 are REE-flat to slightly LREE-enriched (La/SmPM = 1-4) at low HREE abundances (0.5-1.5 ppm Yb). Overall, crystallization orders, mineral and whole-rock chemistry suggest origin of ophiolitic cumulates from low-Ca boninites or primitive andesites (higher Opx or Amph proportion) to high-Ca boninites or primitive island arc tholeiites (Cpx-dominating, Amph-free associations with subordinate Opx). Ophiolitic

  19. P wave velocity of Proterozoic upper mantle beneath central and southern Asia

    NASA Astrophysics Data System (ADS)

    Nyblade, Andrew A.; Vogfjord, Kristin S.; Langston, Charles A.

    1996-05-01

    P wave velocity structure of Proterozoic upper mantle beneath central and southern Africa was investigated by forward modeling of Pnl waveforms from four moderate size earthquakes. The source-receiver path of one event crosses central Africa and lies outside the African superswell while the source-receiver paths for the other events cross Proterozoic lithosphere within southern Africa, inside the African superswell. Three observables (Pn waveshape, PL-Pn time, and Pn/PL amplitude ratio) from the Pnl waveform were used to constrain upper mantle velocity models in a grid search procedure. For central Africa, synthetic seismograms were computed for 5880 upper mantle models using the generalized ray method and wavenumber integration; synthetic seismograms for 216 models were computed for southern Africa. Successful models were taken as those whose synthetic seismograms had similar waveshapes to the observed waveforms, as well as PL-Pn times within 3 s of the observed times and Pn/PL amplitude ratios within 30% of the observed ratio. Successful models for central Africa yield a range of uppermost mantle velocity between 7.9 and 8.3 km s-1, velocities between 8.3 and 8.5 km s-1 at a depth of 200 km, and velocity gradients that are constant or slightly positive. For southern Africa, successful models yield uppermost mantle velocities between 8.1 and 8.3 km s-1, velocities between 7.9 and 8.4 km s-1 at a depth of 130 km, and velocity gradients between -0.001 and 0.001 s-1. Because velocity gradients are controlled strongly by structure at the bottoming depths for Pn waves, it is not easy to compare the velocity gradients obtained for central and southern Africa. For central Africa, Pn waves turn at depths of about 150-200 km, whereas for southern Africa they bottom at ˜100-150 km depth. With regard to the origin of the African superswell, our results do not have sufficient resolution to test hypotheses that invoke simple lithospheric reheating. However, our models are not

  20. A hypothesis for Proterozoic-Phanerozoic supercontinent cyclicity, with implications for mantle convection, plate tectonics and Earth system evolution

    NASA Astrophysics Data System (ADS)

    Grenholm, Mikael; Scherstén, Anders

    2015-11-01

    We present a conceptual model for supercontinent cycles in the Proterozoic-Phanerozoic Eons. It is based on the repetitive behavior of C and Sr isotopes in marine carbonates and U-Pb ages and εHf of detrital zircons seen during the Neoproterozoic-Paleozoic and Paleoproterozoic Eras, respectively. These records are considered to reflect secular changes in global tectonics, and it is hypothesized that the repetitive pattern is caused by the same type of changes in global tectonics. The fundamental premise of this paper is that such repetitive changes should also be recorded in orogenic belts worldwide. This carries the implication that Neoproterozoic-Paleozoic orogenic belts should have Paleoproterozoic equivalents. It is proposed that this is the case for the East African, Uralides and Ouachita-Alleghanian orogens, which have Paleoproterozoic analogs in the West African-Amazon, Laurentian and East European cratons, respectively. The Neoproterozoic-Paleozoic orogenic belts are not isolated features but occur in a specific global context, which correspond to the relatively well-constrained Neoproterozoic break-up of Rodinia, and the subsequent Late Paleozoic assembly of Pangea. The existence of Paleoproterozoic equivalents to Neoproterozoic-Paleozoic orogens requires that the same cycle defined the Paleoproterozoic. We therefore hypothesize that there were Paleoproterozoic supercontinents equivalent to Rodinia and Pangea, and that Proterozoic-Phanerozoic supercontinents are comprised of two basic types of configurations, equivalent to Rodinia (R-type) and Pangea (P-type). The Paleoproterozoic equivalent of Rodinia is likely the first supercontinent to have formed, and Proterozoic-Phanerozoic supercontinent cycles are therefore defined by R- to R-type cycles, each lasting approximately 1.5 Gyr. We use this cyclic pattern as a framework to develop a conceptual model that predicts the configuration and cycles of Proterozoic-Phanerozoic supercontinents, and their

  1. Investigating Interactions between the Silica and Carbon Cycles during Precipitation and Early Diagenesis of Authigenic Clay/Carbonate-Mineral Associations in the Carbonate Rock Record

    NASA Astrophysics Data System (ADS)

    McKenzie, J. A.; Francisca Martinez Ruiz, F.; Sanchez-Roman, M.; Anjos, S.; Bontognali, T. R. R.; Nascimento, G. S.; Vasconcelos, C.

    2017-12-01

    The study of authigenic clay/carbonate-mineral associations within carbonate sequences has important implications for the interpretation of scientific problems related with rock reservoir properties, such as alteration of potential porosity and permeability. More specifically, when clay minerals are randomly distributed within the carbonate matrix, it becomes difficult to predict reservoir characteristics. In order to understand this mineral association in the geological record, we have undertaken a comparative study of specially designed laboratory experiments with modern environments, where clay minerals have been shown to precipitate together with a range of carbonate minerals, including calcite, Mg-calcite and dolomite. Two modern dolomite-forming environments, the Coorong lakes, South Australia and Brejo do Espinho Rio de Janeiro, Brazil, were selected for this investigation. For comparative evaluation, enrichment microbial culture experiments, using natural pore water from Brejo do Espinho as the growth medium to promote mineral precipitation, were performed under both aerobic and anaerobic conditions. To establish the environmental parameters and biological processes facilitating the dual mineral association, the experimental samples have been compared with the natural minerals using HRTEM measurements. The results demonstrate that the clay and carbonate minerals apparently do not co-precipitate, but the precipitation of the different minerals in the same sample has probably occurred under different environmental conditions with variable chemistries, e.g., hypersalinity versus normal salinity resulting from the changing ratio of evaporation versus precipitation. Thus, the investigated mineral association is not a product of diagenetic processes but of sequential in situ precipitation processes related to changes in the silica and carbon availability. Implications for ancient carbonate formations will be presented and discussed in the context of a specific

  2. Geotechnical Descriptions of Rock and Rock Masses.

    DTIC Science & Technology

    1985-04-01

    determined in the field on core speci ns by the standard Rock Testing Handbook Methods . afls GA DTIC TAB thannounod 13 Justifiatlo By Distributin...to provide rock strength descriptions from the field. The point-load test has proven to be a reliable method of determining rock strength properties...report should qualify the reported spacing values by stating the methods used to determine spacing. Preferably the report should make the determination

  3. 3-D inversion of complex magnetotelluric data from an Archean-Proterozoic terrain in northeastern São Francisco Craton, Brazil

    NASA Astrophysics Data System (ADS)

    Bologna, Mauricio S.; Egbert, Gary D.; Padilha, Antonio L.; Pádua, Marcelo B.; Vitorello, Ícaro

    2017-09-01

    We present a magnetotelluric (MT) study in the northeastern part of the São Francisco Craton that encompasses an Archean-Proterozoic terrain, the Serrinha Block, breached by a rift basin developed mostly in Early Cretaceous times during the opening of the South Atlantic Ocean. Even though the MT sites are regularly spaced, the profiles have different orientations from one another, making the data distribution over the area highly uneven and therefore non-ideal for 3-D modeling. However, the data set is very complex, with dimensionality analysis indicating prevalence of 3-D geoelectric structure. Results from 3-D inversion are evaluated for robustness and potentiality for yielding tectonic information. At upper crustal depths, the resulting 3-D model is coherent with surface geology, whereas at mid and lower crustal depths more cryptic structures are revealed, likely of Palaeoproterozoic age. The most striking features in the model are several strong (∼1 Ωṡm) crustal conductors beneath the central part of the Serrinha Block, which we attribute to a Palaeoproterozoic oceanic plate subduction and arc-continent collision event involving the Rio Itapicuru Greenstone Belt and the basement of the Serrinha Block. The west-dipping geometry of these conductors provides a constraint on subduction polarity and gives support to tectonic evolutionary models proposing that the Rio Itapicuru Belt was formed in an island arc environment.

  4. Evaluation of magma mixing in the subvolcanic rocks of Ghansura Felsic Dome of Chotanagpur Granite Gneiss Complex, eastern India

    NASA Astrophysics Data System (ADS)

    Gogoi, Bibhuti; Saikia, Ashima; Ahmad, Mansoor; Ahmad, Talat

    2018-06-01

    The subvolcanic rocks exposed in the Ghansura Felsic Dome (GFD) of the Bathani volcano-sedimentary sequence at the northern fringe of the Rajgir fold belt in the Proterozoic Chotanagpur Granite Gneiss Complex preserves evidence of magma mixing and mingling in mafic (dolerite), felsic (microgranite) and intermediate (hybrid) rocks. Structures like crenulated margins of mafic enclaves, felsic microgranular enclaves and ocelli with reaction surfaces in mafic rocks, hybrid zones at mafic-felsic contacts, back-veining and mafic flows in the granitic host imply magma mingling phenomena. Textural features like quartz and titanite ocelli, acicular apatite, rapakivi and anti-rapakivi feldspar intergrowths, oscillatory zoned plagioclase, plagioclase with resorbed core and intact rim, resorbed crystals, mafic clots and mineral transporting veins are interpreted as evidence of magma mixing. Three distinct hybridized rocks have formed due to varied interactions of the intruding mafic magma with the felsic host, which include porphyritic diorite, mingled rocks and intermediate rocks containing felsic ocelli. Geochemical signatures confirm that the hybrid rocks present in the study area are mixing products formed due to the interaction of mafic and felsic magmas. Physical parameters like temperature, viscosity, glass transition temperature and fragility calculated for different rock types have been used to model the relative contributions of mafic and felsic end-member magmas in forming the porphyritic diorite. From textural and geochemical investigations it appears that the GFD was a partly solidified magma chamber when mafic magma intruded it leading to the formation of a variety of hybrid rock types.

  5. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    NASA Astrophysics Data System (ADS)

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.; Cliff, John B.; Belousova, Elena A.; Sheppard, Stephen

    2017-01-01

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as 'cratonization', is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons from several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons. The majority of magmatic zircons from the main magmatic cycles have Hf isotopic compositions that are generally more evolved than CHUR, forming vertical arrays that extend to moderately radiogenic compositions. Complimentary O isotope data, also show a significant variation in composition. However, combined, these data define not only the source components from which the magmas were derived, but also a range of physio-chemical processes that operated during magma transport and emplacement. These data also identify a previously unknown crustal reservoir in the Capricorn Orogen.

  6. An isotopic perspective on growth and differentiation of Proterozoic orogenic crust: From subduction magmatism to cratonization

    SciT

    Johnson, Simon P.; Korhonen, Fawna J.; Kirkland, Christopher L.

    The in situ chemical differentiation of continental crust ultimately leads to the long-term stability of the continents. This process, more commonly known as ‘cratonization’, is driven by deep crustal melting with the transfer of those melts to shallower regions resulting in a strongly chemically stratified crust, with a refractory, dehydrated lower portion overlain by a complementary enriched upper portion. Since the lower to mid portions of continental crust are rarely exposed, investigation of the cratonization process must be through indirect methods. In this study we use in situ Hf and O isotope compositions of both magmatic and inherited zircons frommore » several felsic magmatic suites in the Capricorn Orogen of Western Australia to highlight the differentiation history (i.e. cratonization) of this portion of late Archean to Proterozoic orogenic crust. The Capricorn Orogen shows a distinct tectonomagmatic history that evolves from an active continental margin through to intracratonic reworking, ultimately leading to thermally stable crust that responds similarly to the bounding Archean Pilbara and Yilgarn Cratons.« less

  7. Terminal proterozoic mid-shelf Benthic microbial mats in the Centralian Superbasin and their environmental significance

    NASA Astrophysics Data System (ADS)

    Logan, Graham A.; Calver, Clive R.; Gorjan, Paul; Summons, Roger E.; Hayes, John M.; Walter, Malcolm R.

    1999-05-01

    A combined sedimentological and biogeochemical study has been conducted on several Terminal Proterozoic mid-shelf microbial mat facies from the Centralian Super-basin. Isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from 'planktonic' and 'benthic microbial-mat' populations contributed to the sediment. The 'planktonic' source provided a suite of n-alkanes with C 20, whereas, the 'benthic' source contributed an overlay of n-alkanes >C 20 with a strong even preference, together with mid-chain methyl alkanes. Kerogen and biomarkers derived from the microbial mat were found to be depleted in 13C relative to planktonic material. Pyrite in the micorbial mats was also found to be depleted in 34S compared to surrounding facies. The combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria. These organisms have specific environmental tolerances that set limits on palaeo-environment. Their requirement for oxygen indicates that the water column above the mid-shelf could not have been anoxic. Accordingly, from the results and age determinations reported here, it would appear that mid-shelf environments of the Centralian Superbasin of Australia were seeing significant levels of oxygen through the Ediacarian.

  8. Sporulation and ultrastructure in a late Proterozoic cyanophyte - Some implications for taxonomy and plant phylogeny

    NASA Technical Reports Server (NTRS)

    Cloud, P.; Moorman, M.; Pierce, D.

    1975-01-01

    Electron microscopical studies of a morphologically diverse, coccoid, presumably late Proterozoic blue-green alga are here reported. They show, together with light microscopy, that the form studied is widespread in the Cordilleran geosyncline, extend the record of well-defined endosporangia perhaps 700 million years into the past, and reveal previously unrecorded ultrastructural details. Coming from northeastern Utah, southwestern Alberta, and east central Alaska, these minute fossils belong to the recently described, morphologically diverse taxon Sphaerocongregus variabilis Moorman, are related to living entophysalidaceans, and have affinities with both the chroococcalean and chamaesiphonalean cyanophytes. Included in the morphological modes displayed by this alga are individual unicells, coenobial clusters of unicells, and a range of endosporangia comparable to those described for living entophysalidaceans. Scanning and transmission electron microscopy reveal that the endospores are commonly embedded in a vesicular matrix, that some of them show what appears to be a bilaminate or perhaps locally multilaminate wall structure, and that some remain together to mature as coenobial clones or 'colonies'. Taxonomic classification and phylogeny are discussed.

  9. Probable calcified metaphytes in the latest Proterozoic Nama Group, Namibia: origin, diagenesis, and implications

    NASA Technical Reports Server (NTRS)

    Grant, S. W.; Knoll, A. H.; Germs, G. J.

    1991-01-01

    Samples from the Huns Limestone Member, Urusis Formation, Nama Group, at two adjacent localities in southern Namibia contain thin foliose to arched, sheet-like carbonate crusts that are 100-500 micrometers thick and up to 5 cm in lateral dimension. Morphologic, petrographic, and geochemical evidence supports the interpretation of these delicate crusts as biogenic, most likely the remains of calcified encrusting metaphytes. The original sediments of the fossiliferous samples contained aragonitic encrusting algae, botryoidal aragonite cements, and an aragonite mud groundmass. Spherulites within the precursor mud could represent bacterially induced mineral growths or the concretions of marine rivularian cyanobacteria. Original textures were severely disrupted during the diagenetic transition of aragonite to low-magnesian calcite, but some primary structures remain discernible as ghosts in the neomorphic mosaic. Gross morphology, original aragonite mineralogy, and hypobasal calcification indicate that the crusts are similar to late Paleozoic phylloid algae and extant peyssonnelid red algae. Structures interpreted as possible conceptacles also suggest possible affinities with the Corallinaceae. Two species of Cloudina, interpreted as the remains of a shelly metazoan, are also known from limestones in the Nama Group. It is possible, therefore, that skeletalization in metaphytes and animals arose nearly simultaneously near the end of the Proterozoic Eon.

  10. Rocks Can Wow? Yes, Rocks Can Wow!

    ERIC Educational Resources Information Center

    Hardman, Sally; Luke, Sue

    2016-01-01

    Rocks and fossils appear in the National Curriculum of England science programmes of study for children in year 3 (ages 7-8). A frequently asked question is "How do you make the classification of rocks engaging?" In response to this request from a school, a set of interactive activities was designed and organised by tutors and students…

  11. Finding the right rocks on Mars

    NASA Astrophysics Data System (ADS)

    Hargraves, R. B.; Knudsen, J. M.; Madsen, M. B.; Bertelsen, P.

    Locating a rock on the surface of Mars that bears unambiguous evidence of the existence—prior or present—of life on that planet is, understandably, the “Holy Grail” of NASAs sample return missions. Remote recognition of such a rock on Mars will not be easy. We do know, however, that present in the Martian crust—especially in the “Southern highlands”—is rock carrying strong natural remanent magnetization (NRM). Characterization of such magnetized rock has profound implications for adding to our knowledge about the origin and early evolution of the Martian interior, lithosphere, atmosphere, and possibly even Martian life forms [Ward and Brownlee, 2000]. Moreover, it should be possible to recognize such rocks by use of a simple magnetic compass mounted on a Rover.

  12. Rocks in Our Pockets

    ERIC Educational Resources Information Center

    Plummer, Donna; Kuhlman, Wilma

    2005-01-01

    To introduce students to rocks and their characteristics, teacher can begin rock units with the activities described in this article. Students need the ability to make simple observations using their senses and simple tools.

  13. Rocks and Minerals.

    ERIC Educational Resources Information Center

    Naturescope, 1987

    1987-01-01

    Provides background information on rocks and minerals, including the unique characteristics of each. Teaching activities on rock-hunting and identification, mineral configurations, mystery minerals, and growing crystals are provided. Reproducible worksheets are included for two of the activities. (TW)

  14. Rock slope design guide.

    DOT National Transportation Integrated Search

    2011-04-01

    This Manual is intended to provide guidance for the design of rock cut slopes, rockfall catchment, and : rockfall controls. Recommendations presented in this manual are based on research presented in Shakoor : and Admassu (2010) entitled Rock Slop...

  15. Rock Slope Design Criteria

    DOT National Transportation Integrated Search

    2010-06-01

    Based on the stratigraphy and the type of slope stability problems, the flat lying, Paleozoic age, sedimentary : rocks of Ohio were divided into three design units: 1) competent rock design unit consisting of sandstones, limestones, : and siltstones ...

  16. The Rock Cycle

    ERIC Educational Resources Information Center

    Singh, Raman J.; Bushee, Jonathan

    1977-01-01

    Presents a rock cycle diagram suitable for use at the secondary or introductory college levels which separates rocks formed on and below the surface, includes organic materials, and separates products from processes. (SL)

  17. Theory of wing rock

    NASA Technical Reports Server (NTRS)

    Hsu, C.-H.; Lan, C. E.

    1985-01-01

    Wing rock is one type of lateral-directional instabilities at high angles of attack. To predict wing rock characteristics and to design airplanes to avoid wing rock, parameters affecting wing rock characteristics must be known. A new nonlinear aerodynamic model is developed to investigate the main aerodynamic nonlinearities causing wing rock. In the present theory, the Beecham-Titchener asymptotic method is used to derive expressions for the limit-cycle amplitude and frequency of wing rock from nonlinear flight dynamics equations. The resulting expressions are capable of explaining the existence of wing rock for all types of aircraft. Wing rock is developed by negative or weakly positive roll damping, and sustained by nonlinear aerodynamic roll damping. Good agreement between theoretical and experimental results is obtained.

  18. Design Support of an Above Cap-rock Early Detection Monitoring System using Simulated Leakage Scenarios at the FutureGen2.0 Site

    DOE PAGES

    Williams, Mark D.; USA, Richland Washington; Vermuel, Vince R.; ...

    2014-12-31

    The FutureGen 2.0 Project will design and build a first-of-its-kind, near-zero emissions coal-fueled power plant with carbon capture and storage (CCS). To assess storage site performance and meet the regulatory requirements of the Class VI Underground Injection Control (UIC) Program for CO 2 Geologic Sequestration, the FutureGen 2.0 project will implement a suite of monitoring technologies designed to evaluate CO 2 mass balance and detect any unforeseen loss in CO 2 containment. The monitoring program will include direct monitoring of the reservoir, and early-leak-detection monitoring directly above the primary confining zone. This preliminary modeling study described here focuses on hypotheticalmore » leakage scenarios into the first permeable unit above the primary confining zone (Ironton Sandstone) and is used to support assessment of early-leak detection capabilities. Future updates of the model will be used to assess potential impacts on the lowermost underground source of drinking water (Saint Peter Sandstone) for a range of theoretical leakage scenarios. This preliminary modeling evaluation considers both pressure response and geochemical signals in the overlying Ironton Sandstone. This model is independent of the FutureGen 2.0 reservoir model in that it does not simulate caprock discontinuities, faults, or failure scenarios. Instead this modeling effort is based on theoretical, volumetric-rate based leakage scenarios. The scenarios include leakage of 1% of the total injected CO 2 mass, but spread out over different time periods (20, 100, and 500 years) with each case yielding a different mass flux (i.e., smaller mass fluxes for longer duration leakage cases]. A brine leakage scenario using a volumetric leakage similar to the 20 year 1% CO 2 case was also considered. A framework for the comparison of the various cases was developed based on the exceedance of selected pressure and geochemical thresholds at different distances from the point of leakage and

  19. Origin of limestone-marlstone cycles: Astronomic forcing of organic-rich sedimentary rocks from the Cenomanian to early Coniacian of the Cretaceous Western Interior Seaway, USA

    NASA Astrophysics Data System (ADS)

    Eldrett, James S.; Ma, Chao; Bergman, Steven C.; Ozkan, Aysen; Minisini, Daniel; Lutz, Brendan; Jackett, Sarah-Jane; Macaulay, Calum; Kelly, Amy E.

    2015-08-01

    We present an integrated multidisciplinary study of limestone-marlstone couplets from a continuously cored section including parts of the upper Buda Limestone, the entire Eagle Ford Group (Boquillas Formation) and lower Austin Chalk from the Shell Iona-1 research borehole (Texas, USA), which provides a >8 million year (myr) distal, clastic sediment-starved, intrashelf basin record of the early Cenomanian to the earliest Coniacian Stages. Results show that despite variable yet minimal diagenetic overprints, several unambiguous primary environmental signals are preserved and support greater water-mass ventilation and current activity promoting increased silica/carbonate productivity during the deposition of limestone beds compared to deposition of marlstone beds which reflect greater organic matter productivity and preservation. Furthermore, our astronomical analyses demonstrate that the limestone-marlstone couplets in the Iona-1 core reflect climatic forcing driven by solar insolation resulting from integrated Milankovitch periodicities. In particular, we propose that obliquity and precession forcing on the latitudinal distribution of solar insolation may have been responsible for the observed lithological and environmental variations through the Cenomanian, Turonian and Coniacian in this mid-latitude epicontinental sea setting. Our data also suggests that rhythmic lithological alternations deposited in Greenhouse periods, in general, may simply reflect climate-driven cycles related to Earth-Sun dynamics without the need to invoke significant sea-level variations.

  20. Early Tertiary Exhumation, Erosion, and Sedimentation in the Central Andes, NW Argentina

    NASA Astrophysics Data System (ADS)

    Carrapa, B.; Decelles, P. G.; Gerhels, G.; Mortimer, E.; Strecker, M. R.

    2006-12-01

    Timing of deformation and resulting sedimentation patterns in the Altiplano-Puna Plateau-Eastern Cordillera of the southern Central Andes are the subject of ongoing controversial debate. In the Bolivian Altiplano, sedimentation into a foreland basin system commenced during the Paleocene. Farther south in the Puna and Eastern Cordillera of NW Argentina, a lack of data has precluded a similar interpretation. Early Tertiary non-marine sedimentary rocks are preserved within the present day Puna Plateau and Eastern Cordillera of NW Argentina. The Salar de Pastos Grandes basin in the Puna Plateau contains more than 2 km of Eocene alluvial and fluvial strata in the Geste Formation, deposited in close proximity to orogenic source terrains. Sandstone and conglomerate petrographic data document Ordovician quartzites and minor phyllites and schists as the main source rocks. Detrital zircon U-Pb ages from both the Geste Formation and from underlying Ordovician quartzite cluster in the 900-1200 Ma (Grenville) and late Precambrian-Cambrian (Panafrican) ranges. Sparse late Eocene (~37-34 Ma) grains are also present; their large size, euhedral shape, and decreasing mean ages upsection suggest that these grains are volcanogenic (i.e. ash fall contamination), derived from an inferred magmatic arc to the west. The Eocene ages corroborate mammalian paleontological dates, defining the approximate begin of deposition of the Geste Formation. Alternatively, these young zircons could be of plutonic origin; however, no Eocene plutons are present in the surrounding source rocks and this interpretation is not likely. From W to E, fluvial rocks of the Quebrada de los Colorados Formation show similar sedimentological features as those observed for the Geste Formation, suggesting a genetic link between the two. Detrital zircon U-Pb data show mainly Panafrican ages, with sparse ages in the 860-935 Ma range and a few mid-Proterozoic ages. More importantly, a significant number of late Eocene

  1. 68. LITTLE ROCK AND PALMDALE IRRIGATION DISTRICT, LITTLE ROCK DAM: ...

    Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

    68. LITTLE ROCK AND PALMDALE IRRIGATION DISTRICT, LITTLE ROCK DAM: STRESS SHEET, SHEET 4; MAY, 1918. Littlerock Water District files. - Little Rock Creek Dam, Little Rock Creek, Littlerock, Los Angeles County, CA

  2. My Pet Rock

    ERIC Educational Resources Information Center

    Lark, Adam; Kramp, Robyne; Nurnberger-Haag, Julie

    2008-01-01

    Many teachers and students have experienced the classic pet rock experiment in conjunction with a geology unit. A teacher has students bring in a "pet" rock found outside of school, and the students run geologic tests on the rock. The tests include determining relative hardness using Mohs scale, checking for magnetization, and assessing luster.…

  3. Evidence of early Archean crust in northwest Gondwana, from U-Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

    NASA Astrophysics Data System (ADS)

    Naidoo, Thanusha; Zimmermann, Udo; Vervoort, Jeff; Tait, Jenny

    2018-03-01

    The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U-Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U-Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565-633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636-996 Ma); and (2) smaller Mesoproterozoic (5%; 1004-1240 Ma), Palaeoproterozoic (11%; 1890-2476 Ma) and Archean (11%; 2519-3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3-1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial ɛHf values between -15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (ɛHf(i) +10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative ɛHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

  4. Archean Arctic continental crust fingerprints revealing by zircons from Alpha Ridge bottom rocks

    NASA Astrophysics Data System (ADS)

    Sergeev, Sergey; Petrov, Oleg; Morozov, Andrey; Shevchenko, Sergey; Presnyakov, Sergey; Antonov, Anton; Belyatsky, Boris

    2015-04-01

    Whereas thick Cenozoic sedimentary cover overlapping bedrock of the Arctic Ocean, some tectonic windows were sampled by scientific submarine manipulator, as well as by grabbing, dredging and drilling during «Arctic-2012» Russian High-Arctic expedition (21 thousands samples in total, from 400-km profile along Alpha-Mendeleev Ridges). Among others, on the western slope of Alpha Ridge one 10x10 cm fragment without any tracks of glacial transportation of fine-layered migmatitic-gneiss with prominent quartz veinlets was studied. Its mineral (47.5 vol.% plagioclase + 29.6% quartz + 16.6% biotite + 6.1% orthoclase) and chemical composition (SiO2:68.2, Al2O3:14.9, Fe2O3:4.44, TiO2:0.54, MgO:2.03, CaO:3.13, Na2O:3.23, K2O:2.16%) corresponds to trachydacite vulcanite, deformed and metamorphozed under amphibolite facies. Most zircon grains (>80%) from this sample has an concordant U-Pb age 3450 Ma with Th/U 0.8-1.4 and U content of 100-400 ppm, epsilon Hf from -4 up to 0, and ca 20% - ca 3.3 Ga with Th/U 0.7-1.4 and 90-190 ppm U, epsilon Hf -6.5 to -4.5, while only 2% of the grains show Proterozoic age of ca 1.9 Ga (Th/U: 0.02-0.07, U~500 ppm, epsilon Hf about 0). No younger zircons were revealed at all. We suppose that magmatic zircon crystallized as early as 3450 Ma ago during acid volcanism, the second phase zircon crystallization from partial melt (or by volcanics remelting) under amphibolite facies metamorphism was at 3.3 Ga ago with formation of migmatitie gneisses. Last zircon formation from crustal fluids under low-grade metamorphic conditions was 1.9 Ga ago. There are two principal possibilities for the provenance of this metavolcanic rock. The first one - this is ice-rafted debris deposited by melted glacial iceberg. However, presently there are no temporal and compositional analogues of such rocks in basement geology of peri-oceanic regions, including Archean Itsaq Gneiss Complex, Lewisian Complex and Baltic Shield but these regions are far from the places of

  5. Maps showing geology, structure, and geophysics of the central Black Hills, South Dakota

    Redden, Jack A.; DeWitt, Ed

    2008-01-01

    This 1:100,000-scale digital geologic map details the complex Early Proterozoic granitic rocks, Early Proterozoic supracrustal metamorphic rocks, and Archean crystalline basement of the Black Hills. The granitic rocks host pegmatite deposits renowned for their feldspar, mica, spodumene, and beryl. The supracrustal rocks host the Homestake gold mine, which produced more than 40 million ounces of gold over a 125-year lifetime. The map documents the Laramide deformation of Paleozoic and Mesozoic cover rocks; and shows the distribution of Laramide plutonic rocks associated with precious-metals deposits. Four 1:300,000-scale maps summarize Laramide structures; Early Proterozoic structures; aeromagnetic anomalies; and gravity anomalies. Three 1:500,000-scale maps show geophysical interpretations of buried Early Proterozoic to Archean rocks in western South Dakota and eastern Wyoming.

  6. Transportable seismic array tomography in southeast Australia: Illuminating the transition from Proterozoic to Phanerozoic lithosphere

    NASA Astrophysics Data System (ADS)

    Rawlinson, N.; Salmon, M.; Kennett, B. L. N.

    2014-02-01

    The Phanerozoic Tasmanides of eastern Australia is comprised of a series of orogenic belts that developed along the east margin of Gondwana following the breakup of the supercontinent Rodinia and subsequent formation of the Pacific Ocean. The tectonic complexities of this region have been well studied, but most work has been confined to evidence collected from the near surface, where extensive Mesozoic and Cenozoic basin cover masks large tracts of Palaeozoic basement. We apply teleseismic tomography to distant earthquake data recorded by WOMBAT - the largest transportable seismic array experiment in the southern hemisphere - to image P-wavespeed variations in the mantle lithosphere beneath the southern portion of the Tasmanides in detail. In order to seamlessly suture together the teleseismic datasets from each of the 14 sub-arrays of WOMBAT, we use P-wavespeeds from the AuSREM model to construct a laterally heterogeneous starting model that captures the long wavelength structural variations that would otherwise be lost through the use of relative arrival time residuals. Synthetic resolution tests indicate good horizontal resolution of ~ 50 km within most of the array between depths of 50-350 km. A key feature of the 3-D P-wave model is a pronounced easterly high velocity salient in the mantle lithosphere beneath the northern limit of the New England Orocline, which may indicate the presence of underpinning Proterozoic lithosphere that was instrumental in its formation. Another pronounced high velocity anomaly underlies the Curnamona Province, a large crustal block with a strong Archean provenance, which is clearly separated from the Gawler Craton to the west at upper mantle depths by a low velocity zone beneath the Adelaide Fold Belt. We also estimate the location of the eastern boundary of Precambrian Australia at depth, and show that it extends eastward further than previously thought.

  7. Microbial communities and organic biomarkers in a Proterozoic-analog sinkhole.

    PubMed

    Hamilton, T L; Welander, P V; Albrecht, H L; Fulton, J M; Schaperdoth, I; Bird, L R; Summons, R E; Freeman, K H; Macalady, J L

    2017-11-01

    Little Salt Spring (Sarasota County, FL, USA) is a sinkhole with groundwater vents at ~77 m depth. The entire water column experiences sulfidic (~50 μM) conditions seasonally, resulting in a system poised between oxic and sulfidic conditions. Red pinnacle mats occupy the sediment-water interface in the sunlit upper basin of the sinkhole, and yielded 16S rRNA gene clones affiliated with Cyanobacteria, Chlorobi, and sulfate-reducing clades of Deltaproteobacteria. Nine bacteriochlorophyll e homologues and isorenieratene indicate contributions from Chlorobi, and abundant chlorophyll a and pheophytin a are consistent with the presence of Cyanobacteria. The red pinnacle mat contains hopanoids, including 2-methyl structures that have been interpreted as biomarkers for Cyanobacteria. A single sequence of hpnP, the gene required for methylation of hopanoids at the C-2 position, was recovered in both DNA and cDNA libraries from the red pinnacle mat. The hpnP sequence was most closely related to cyanobacterial hpnP sequences, implying that Cyanobacteria are a source of 2-methyl hopanoids present in the mat. The mats are capable of light-dependent primary productivity as evidenced by 13 C-bicarbonate photoassimilation. We also observed 13 C-bicarbonate photoassimilation in the presence of DCMU, an inhibitor of electron transfer to Photosystem II. Our results indicate that the mats carry out light-driven primary production in the absence of oxygen production-a mechanism that may have delayed the oxygenation of the Earth's oceans and atmosphere during the Proterozoic Eon. Furthermore, our observations of the production of 2-methyl hopanoids by Cyanobacteria under conditions of low oxygen and low light are consistent with the recovery of these structures from ancient black shales as well as their paucity in modern marine environments. © 2017 The Authors. Geobiology Published by John Wiley & Sons Ltd.

  8. Geology and origin of the late Proterozoic Darb Zubaydah ophiolite, Kingdom of Saudi Arabia

    Quick, J.E.

    1990-01-01

    The Darb Zubaydah ophiolite, north-central Arabian Shield, preserves a largely intact section consisting of ultramafic rocks, gabbro, diabase, granodiorite, and interbedded volcanic and sedimentary rocks. Formation of these rocks within or near an island arc is indicated by the absence of pelagic sediments and the abundance of pillow basalt, turbiditic sediments, lahar deposits, and basaltic to rhyolitic tuff. The oldest extrusive rocks formed in a young, relatively unevolved island arc or in a back-arc basin sufficiently close to an arc to receive calc-alkaline lava flows and coarse-grained, arc-derived detritus. Overlying turbidites and lahar deposits of the Kaffan sandstone point to the initiation of a rifting event. High-Ti basalts, which erupted above the Kaffan sandstone, and related diabase are interpreted to be magmatic products of incipient intra-arc rifting. Renewed arc volcanism produced calc-alkaline volcanic rocks that interfingered with the high-Ti basalt and later dominated the section as the volcanic apron of the arc prograded basinward. Extrusion of voluminous calc-alkaline tuff may have been contemporaneous with intrusion of granodiorite and gravity-driven landsliding. -from Author

  9. Rock ramp design guidelines

    Mooney, David M.; Holmquist-Johnson, Christopher L.; Broderick, Susan

    2007-01-01

    Rock ramps or roughened channels consist of steep reaches stabilized by large immobile material (riprap). Primary objectives for rock ramps include: Create adequate head for diversionMaintain fish passage during low-flow conditionsMaintain hydraulic conveyance during high-flow conditionsSecondary objectives for rock ramp design include:Emulate natural systemsMinimize costsThe rock ramp consists of a low-flow channel designed to maintain biologically adequate depth and velocity conditions during periods of small discharges. The remainder of the ramp is designed to withstand and pass large flows with minimal structural damage. The following chapters outline a process for designing rock ramps.

  10. Rb-Sr-analyses of apollo 16 melt rocks and a new age estimate for the imbrium basin: lunar basin chronology and the early heavy bombardment of the moon

    SciT

    Deutsch, A.; Stoeffler, D.

    1987-07-01

    Rb-Sr-model ages on 7 impact glass-bombs and internal Rb-Sr isochrons for two crystalline impact melt rocks from the Apollo 16 collection have been determined. The post-Cayley glass-bombs with model ages between 4.75 +- 0.45 AE and 3.97 +- 0.08 AE can be classified according to their calculated single stage (/sup 87/Rb/sup 86/Sr)/sub I/-ratios: 67728, 67946, and 67627.8 point to a KREEP-free precursor terrain - the Descartes highlands; whereas 63566, 67567, 67627.10 and 67629 are derived from the more heterogeneous Cayley plains. The very feldspar-rich impact melt rock 65795, which is compositionally similar to the group of feldspathic microporphyritic melt brecciasmore » (FM-suite), yields a crystallization age of 3.81 +- 0.04 AE (2sigma; lambda/sup 87/Rb = 1.42/sup -11/ yr/sup -1/) and I/sub Sr/ of .69929 +- 3. The authors suggest that the Imbrium basin and the related Fra Mauro and Cayley formations were formed 3.77 +- 0.02 AE ago and could be even as young as 3.75 AE. As a consequence, they adopt 3.92 +- 0.03 AE, 3.87 +- 0.03 AE, and 3.84 +- 0.04 AE as ages for the Nectaris, Serenitatis, and Crisium basins, respectively, in agreement with the relative crater densities measured on the ejecta blankets of these basins. The proposed age sequence leads to an average formation interval for the observed 12-13 Nectarian basins of 7 to 14 m.y. leaving approx. 30 pre-Nectarian basins of unknown age. These facts suggest that there is no late terminal lunar cataclysm in the sense of a culmination of the lunar impact rate at approx. 3.8 AE ago. Rather, the observations are compatible with a steeply and steadily decreasing flux of impactors in the sense of an early heavy bombardment which started at the time of the moon's accretion and terminated around 3.75 AE ago.« less

  11. Discrimination of alkalinity in granitoid Rocks: A potential TIMS application

    NASA Technical Reports Server (NTRS)

    Ruff, Steven W.

    1995-01-01

    In mineral exploration, the ability to distinguish and map petrochemical variations of magmatic rocks can be a useful reconnaissance tool. Alkalinity is one such petrochemical parameter and is used in the characterization of granitoid rocks. In quartz normative plutonic rocks, alkalinity is related to the composition and abundance of feldspars. Together with quartz abundance, knowledge of feldspar modes allows the classification of these igneous rocks according to the Streckeisen diagram. Alternative classification schemes rely on whole rock geochemistry instead of mineral identifications. The relative ease of obtaining whole rock analyses means that geochemical classifications tend to be favored in exploration geology. But the technique of thermal infrared spectroscopy of rocks yields information on mineralogy and is one that can be applied remotely. The goal of the current work then is to establish whether data from TIMS can be used to distinguish the mineralogical variations that relate to alkalinity. An ideal opportunity to test this thesis arises from the work presented in a paper by Dewitt (1989). This paper contains the results of mapping and analysis of Proterozoic plutonic rocks in north-central Arizona. The map resulting from this work delineates plutons according to alkalinity in an effort to establish a trend or polarity in the regional magmatism. Also contained within this paper are brief descriptions of the mineralogy of half of the region's plutons. This combination of mineralogical and geochemical information was the rationale behind choosing this area as a site for TIMS over flights. A portion of the region centered on the northern Bradshaw Mountains was selected because it contains plutons of all three alkalinity classifications (alkali-calcic, calc-alkalic, and calic) present on DeWitt's map within a relatively small area. The site was flown in August of 1994 and the data received a few days before the writing of this manuscript. Most of this

  12. Stratigraphic records of paleogeography and global change from two late Proterozoic basins

    NASA Astrophysics Data System (ADS)

    Swanson-Hysell, Nicholas L.

    transition. This result allows the magnetizations of the lavas to be fully interpreted, and also suggests that this self-reversal phenomena may be more widespread than currently recognized---with its identification in this study being greatly aided by stratigraphic context during a period when North America was moving rapidly towards the equator. Stratigraphic and stable isotope work on the Neoproterozoic Bitter Springs Formation of the Amadeus Basin demonstrates that the negative carbon isotope values of the "Bitter Springs Stage" are tightly consistent in carbonate rocks across more than 400 km. In addition to being present in the isotopic composition of the carbonate, organic carbon isotope values shift sympathetically into and out of the stage thereby supporting the interpretation that the stage is a record of primary changes to the carbon cycle. The stage is bound by sequence boundaries that provide evidence for changes in sea-level and climate. Previous work on correlative stratigraphy from the Akademikerbreen Group of East Svalbard (Maloof et al., 2006), revealed changes in relative sea-level and paleomagnetic directions that have were interpreted to have resulted from a pair of large-scale true polar wander events. In an effort to further test this hypothesis, and to remedy a lack of paleogeographic constraints for north Australia in the early Neoproterozoic, I present paleomagnetic data from more than 630 paleomagnetic samples of carbonates, siltstones and basalt flows from the Bitter Springs Formation. A new reliable pole from post-Bitter Springs Stage siltstones provides strong support for a recently published hypothesis that there was relative rotation between north and south+west Australia in the late Neoproterozoic (Li and Evans, 2011), and for the long-standing hypothesis that Australia and Laurentia were cotravelers in Rodinia into the mid-Neoproterozoic Era. The difference between the paleomagnetic poles of syn-Bitter Springs Stage carbonates and post

  13. Late Proterozoic glacially controlled shelf sequences in western Mali (west Africa)

    NASA Astrophysics Data System (ADS)

    Deynoux, M.; Prousti, J. N.; Simon, B.

    The Late Proterozoic deposits of the Bakoye Group (500 m) in western Mali constitute a remarkable example of a glacially influenced sedimentary record on an epicratonic platform. They are composed of alternating marine and continental formations which represent accumulation in a basin located in the vicinity of upland areas covered by ice sheets. One of these formations (the Ba4 Formation), which is the focus of this study, is composed of three major units. The basal Unit 1 is made up of carbonaceous coarse to fine grained sandstones which are organized in fining upward sequences and which comprise lenticular diamictite intercalations. This Unit is considered to represent the fore slope gravity flows of a subaqueous ice-cootact fan fed by meltwater streams (≪glacioturbidites≫). Unit 2 is made up of coarse to fine grained sandstones in a highly variable association of facies. This Unit is characterized by the abundance of wave ripples associated with convolute beddings. planar or wavy beddings and tabular or hummocky crossbeddings in a general shallowing upward trend. It also comprises evidence of gravity processes including debris flows and large slumped sandstone bodies. Unit 2 represents the progressive filling of the Ba4 basin and reflects the combined effect of glacially induced eustatism and isostacy during a phase of glacial retreat. The basal part of Unit 3 is made up of a succession (a few meters thick) of conglomerates, diamictites, sandstones, siltstones or carbonates lying on an erosional unconformity marked by periglacial frost wedges. The upper part of Unit 3 is thicker (100-150 m) and onlaps on these basal facies with a succession of sandstone bars exhibiting swaley and hummocky crossbeddings, large cut and fill structures, and planar laminations. Unit 3 is strongly transgressive, the lower shoreface and backshore deposits include algal mats and are onlapped by sand ridges emplaced in a high energy upper to middle shoreface environment. Overall

  14. Mysterious Roving Rocks of Racetrack Playa

    2017-12-08

    Some of the moving rocks are large. This one is about 10 inches tall. Researchers in the late 1960s and early 1970s documented the movements of one very large rock that they named Karen. (The two men named all the rocks after women.) They estimated that Karen weighed 700 pounds. Credit: NASA/GSFC/Maggie McAdam To read a feature story on the Racetrack Playa go to: www.nasa.gov/topics/earth/features/roving-rocks.html NASA Goddard Space Flight Center is home to the nation's largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe. Follow us on Twitter Join us on Facebook

  15. Constraining the location of the Archean--Proterozoic suture in the Great Basin based on magnetotelluric soundings

    Rodriguez, Brian D.; Sampson, Jay A.

    2012-01-01

    It is important to understand whether major mining districts in north-central Nevada are underlain by Archean crust, known to contain major orogenic gold deposits, or, alternatively, by accreted crust of the Paleoproterozoic Mojave province. Determining the location and orientation of the Archean-Proterozoic suture zone between the Archean crust and Mojave province is also critical because it may influence subsequent patterns of sedimentation, deformation, magmatism, and hydrothermal activity. In the Great Basin, the attitude of the suture zone is unknown because it is concealed below cover. A regional magnetotelluric sounding profile along the Utah-Nevada State line reveals a deeply penetrating, broad electrical conductor that may be the Archean-Proterozoic suture zone in the northwest corner of Utah. This major crustal conductor's strike direction is northwest, where it broadens to about 80 km wide below about 3-km depth. These results suggest that the southwestern limit of intact Archean crust in this part of the Great Basin is farther north than previously reported. These results also suggest that the major gold belts in north-central Nevada are located over the Paleoproterozoic Mojave province, and the Archean terrain lies northeast in the northwest corner of Utah. Rifted Archean crust segments south and west of the suture suggest that future mineral exploration northeast of current mineral trends may yield additional gold deposits.

  16. Bounce Rock Dimple

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This panoramic camera image shows the hole drilled by the Mars Exploration Rover Opportunity's rock abrasion tool into the rock dubbed 'Bounce' on Sol 65 of the rover's journey. The tool drilled about 7 millimeters (0.3 inches) into the rock and generated small piles of 'tailings' or rock dust around the central hole, which is about 4.5 centimeters (1.7 inches) across. The image from sol 66 of the mission was acquired using the panoramic camera's 430 nanometer filter.

  17. Hungry for Rocks

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This image from the Mars Exploration Rover Spirit hazard identification camera shows the rover's perspective just before its first post-egress drive on Mars. On Sunday, the 15th martian day, or sol, of Spirit's journey, engineers drove Spirit approximately 3 meters (10 feet) toward its first rock target, a football-sized, mountain-shaped rock called Adirondack (not pictured). In the foreground of this image are 'Sashimi' and 'Sushi' - two rocks that scientists considered investigating first. Ultimately, these rocks were not chosen because their rough and dusty surfaces are ill-suited for grinding.

  18. Geophysical Investigations of a Proterozoic Carbonatite Terrane, southeast Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Denton, K. M.; Ponce, D. A.; Miller, D. M.; Peacock, J.; Miller, J. S.

    2015-12-01

    One of the world's largest rare-earth element-rich carbonatite deposits is located in the eastern Mojave Desert at Mountain Pass, California. The eastern Mojave Desert carbonatite terrane consists of a ~1.7 Ga gneiss and schist rocks that are host to a ~1.417 Ga (Premo, 2013) ultrapotassic intrusive suite (shonkinite, syenite, and granite) and a ~1.375 Ga (DeWitt, 1983) carbonatite deposit . Regional geophysical data indicate that this carbonatite terrane occurs within a north-northwest trending ~1-km wide bench in a gravity high and along the eastern edge of a prominent magnetic high in the eastern Clark Mountain Range. To improve our understanding of the geophysical and structural framework of the eastern Mojave carbonatite terrane, we collected over 2,300 gravity stations and over 640 physical rock property samples. Carbonatite rocks typically have distinct gravity, magnetic, and radioactive signatures because they are relatively dense, often contain magnetite, and are commonly enriched in thorium and/or uranium. Contrary to this trend, our results show that the carbonatite deposit is essentially nonmagnetic with an average susceptibility of 0.18 x 10-3 SI (n=31), and the ultrapotassic intrusive suite is very weakly magnetic with an average susceptibility of 2.0 x 10-3 SI (n=36). However, these rocks are found along a steep gradient of a prominent aeromagnetic anomaly. The lack of magnetic signature from the rocks of the eastern Mojave carbonatite terrane suggests alteration of magnetic minerals. This is corroborated by its location within a broader alteration zone and observed magnetic low. If so, such an alteration event occurred after emplacement of the carbonatite deposit, which likely remobilized rare earth elements in the surrounding rocks. Further, an alteration event is consistent with geology, high rare-earth element concentration, and unusual geochemistry of the carbonatite deposit. Temporal constraints (DeWitt, 1987; Premo, 2013) also suggest

  19. U-Pb zircon and CHIME monazite dating of granitoids and high-grade metamorphic rocks from the Eastern and Peninsular Thailand - A new report of Early Paleozoic granite

    NASA Astrophysics Data System (ADS)

    Kawakami, T.; Nakano, N.; Higashino, F.; Hokada, T.; Osanai, Y.; Yuhara, M.; Charusiri, P.; Kamikubo, H.; Yonemura, K.; Hirata, T.

    2014-07-01

    In order to understand the age and tectonic framework of Eastern to Peninsular Thailand from the viewpoint of basement (metamorphic and plutonic) geology, the LA-ICP-MS U-Pb zircon dating and the chemical Th-U-total Pb isochron method (CHIME) monazite dating were performed in the Khao Chao, Hub-Kapong to Pran Buri, and Khanom areas in Eastern to Peninsular Thailand. The LA-ICP-MS U-Pb zircon dating of the garnet-hornblende gneiss from the Khao Chao area gave 229 ± 3 Ma representing the crystallization age of the gabbro, and that of the garnet-biotite gneisses gave 193 ± 4 Ma representing the timing of an upper amphibolite facies metamorphism. The CHIME monazite dating of pelitic gneiss from the Khao Chao gneiss gave scattered result of 68 ± 22 Ma, due to low PbO content and rejuvenation of older monazite grains during another metamorphism in the Late Cretaceous to Tertiary time. The U-Pb ages of zircon from the Hua Hin gneissic granite in the Hub-Kapong to Pran Buri area scatter from 250 Ma to 170 Ma on the concordia. Granite crystallization was at 219 ± 2 Ma, followed by the sillimanite-grade regional metamorphism at 185 ± 2 Ma. Monazite in the pelitic gneiss from this area also preserves Early to Middle Jurassic metamorphism and rejuvenation by later contact metamorphism by non-foliated granite or by another fluid infiltration event in the Late Cretaceous to Tertiary time. The Khao Dat Fa granite from the Khanom area of Peninsular Thailand gave a U-Pb zircon age of 477 ± 7 Ma. This is the second oldest granite pluton ever reported from Thailand, and is a clear evidence for the Sibumasu block having a crystalline basement that was formed during the Pan-African Orogeny. The Khao Pret granite gives U-Pb zircon concordia age of 67.5 ± 1.3 Ma, which represents the timing of zircon crystallization from the granitic melt and accompanied sillimanite-grade contact metamorphism against surrounding metapelites and gneisses. Metamorphic rocks in the Doi Inthanon area

  20. U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province

    Moscati, Richard J.; Premo, Wayne R.; Dewitt, Ed; Wooden, Joseph L.

    2017-01-01

    A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America.Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation.Our detailed zircon and whole-rock data support the following three interpretations:(1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the

  1. Chocolate Hills Rock

    2010-02-16

    This false-color image, taken by the panoramic camera on NASA rover Opportunity, shows the rock Chocolate Hills, perched on the rim of the 10-meter 33-foot wide Concepcion crater. This rock has a thick, dark-colored coating resembling chocolate.

  2. Welcome to Rock Day

    ERIC Educational Resources Information Center

    Varelas, Maria; Benhart, Jeaneen

    2004-01-01

    At the beginning of the school year, the authors, a first-grade teacher and a teacher educator, worked together to "spice up" the first-grade science curriculum. The teacher had taught the unit Rocks, Sand, and Soil several times, conducting hands-on explorations and using books to help students learn about properties of rocks, but she felt the…

  3. Rock Cycle Roulette.

    ERIC Educational Resources Information Center

    Schmidt, Stan M.; Palmer, Courtney

    2000-01-01

    Introduces an activity on the rock cycle. Sets 11 stages representing the transitions of an earth material in the rock cycle. Builds six-sided die for each station, and students move to the stations depending on the rolling side of the die. Evaluates students by discussing several questions in the classroom. Provides instructional information for…

  4. Rock Bites into 'Bounce'

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This panoramic camera image from the Mars Exploration Rover Opportunity features the 6.44 millimeter (0.25 inch) deep hole ground into the rock dubbed 'Bounce' by the rover's rock abrasion tool. The tool took 2 hours and 15 minutes to grind the hole on sol 66 of the rover's journey. A combination of limited solar power and the rock's jagged texture led the rock abrasion tool team to set very aggressive grinding parameters to ensure that the end result was a full circle, suitable for a thorough read from the rover's spectrometers.

    Bounce's markedly different appearance (when compared to the rocks that were previously examined in the Eagle Crater outcrop) made it a natural target for rover research. In order to achieve an ideal position from which to grind into the rock, Opportunity moved in very close with its right wheel next to Bounce. In this image, the panoramic camera on the rover's mast is looking down, catching the tip of the solar panel which partially blocks the full circle ground by the rock abrasion tool.

    The outer ring consists of the cuttings from the rock, pushed out by the brushes on the grinding instrument. The dark impression at the top of the outer circle was caused by the instrument's contact mechanism which serves to stabilize it while grinding.

  5. Mars Rock Analysis Briefing

    2013-03-12

    David Blake, principal investigator for Curiosity's Chemistry and Mineralogy investigation at NASA's Ames Research Center in Calif., speaks at a news conference presenting findings of the Curiosity rover's analysis of the first sample of rock powder collected on Mars, Tuesday, March 12, 2013 in Washington. The rock sample collected shows ancient Mars could have supported living microbes. Photo Credit: (NASA/Carla Cioffi)

  6. Proterozoic evolution of part of the Embu Complex, eastern São Paulo state, SE Brazil

    NASA Astrophysics Data System (ADS)

    Vinagre da Costa, Rodrigo; Trouw, Rudolph Allard Johannes; Mendes, Julio Cezar; Geraldes, Mauro; Távora, Arthur; Nepomuceno, Felipe; de Araújo Junior, Edson Barros

    2017-11-01

    This paper presents detrital zircon ages obtained in rocks of the Embu Complex, southeastern São Paulo State, Brazil. The Embu Complex encompasses a Paleoproterozoic basement represented by migmatitic hornblende and biotite orthogneisses covered by (kyanite)-(sillimanite)-(garnet) bearing biotite-muscovite schists and paragneisses with decametric intercalations of quartzites and calcsilicate rocks. In the studied area this metasedimentary sequence is intruded by the porphyritic Serra do Quebra Cangalhagranite. Through field and microstructural studies, four ductile deformational phases wereidentified. Metamorphic events related to the Brasiliano Orogeny that affected the studied rocks were dominantly under medium temperature and pressure conditions, from greenschist to middle amphibolite facies. Detrital zircon crystals from a ∼10 m thick quartzite layer were analyzed by LA-ICP-MS resulting in a wide range of ages between 2100 and 600 Ma that fall mainly in four groups: the first group between 2.1 and 1.6 Ga, with apex at 1.7 Ga; two less expressive Mesoproterozoic groups with values between 1.6 and 1.2 Ga; and a fourth group with values between 1.2 and 0.6 Ga. Considering the geochronological data, the sedimentation of the basin began after 852 ± 40 Ma (the youngest igneous grain) and finished before ∼786 Ma (metamorphic rim). The age of the intrusive Serra do Quebra Cangalha granite (∼680 Ma) is consistent with this minimum age. The opening of the basin could be related to the break-up of Rodinia, which resulted in several small continents, among them the Paranapanema and São Francisco paleocontinents. Comparing these data with similar provenance data from the Apiaí terrane, itseems probable that the Embu Complex was physically connected with it during most of their evolution.

  7. Evolution of atmospheric xenon and other noble gases inferred from Archean to Paleoproterozoic rocks

    NASA Astrophysics Data System (ADS)

    Avice, G.; Marty, B.; Burgess, R.; Hofmann, A.; Philippot, P.; Zahnle, K.; Zakharov, D.

    2018-07-01

    We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geological time to shape the modern atmosphere. Modern atmospheric xenon is enriched in heavy isotopes by 30-40‰ u-1 relative to Solar or Chondritic xenon. Previous studies demonstrated that, 3.3 Ga ago, atmospheric xenon was isotopically fractionated (enriched in the light isotopes) relative to the modern atmosphere, by 12.9 ± 1.2 (1σ) ‰ u-1, whereas krypton was isotopically identical to modern atmospheric Kr. Details about the specific and progressive isotopic fractionation of Xe during the Archean, originally proposed by Pujol et al. (2011), are now well established by this work. Xe isotope fractionation has evolved from 21‰ u-1 at 3.5 Ga to 12.9‰ u-1 at 3.3 Ga. The current dataset provides some evidence for stabilization of the Xe fractionation between 3.3 and 2.7 Ga. However, further studies will be needed to confirm this observation. After 2.7 Ga, the composition kept evolving and reach the modern-like atmospheric Xe composition at around 2.1 Ga ago. Xenon may be the second atmospheric element, after sulfur, to show a secular isotope evolution during the Archean that ended shortly after the Archean-Proterozoic transition. Fractionation of xenon indicates that xenon escaped from Earth, probably as an ion, and that Xe escape stopped when the atmosphere became oxygen-rich. We speculate that the Xe escape was enabled by a vigorous hydrogen escape on the early anoxic Earth. Organic hazes, scavenging isotopically heavy Xe, could also have played a role in the evolution of atmospheric Xe. For 3.3 Ga-old samples, Ar-N2 correlations are consistent with a partial pressure of nitrogen (pN2) in the Archean atmosphere

  8. Early Neogene unroofing of the Sierra Nevada de Santa Marta along the Bucaramanga -Santa Marta Fault

    NASA Astrophysics Data System (ADS)

    Piraquive Bermúdez, Alejandro; Pinzón, Edna; Bernet, Matthias; Kammer, Andreas; Von Quadt, Albrecht; Sarmiento, Gustavo

    2016-04-01

    source for sediments, this shift in provenance is related to an increase in exhumation and erosion rates. The instauration of such a highly erosive regime since the Upper Oligocene attests how the Santa Marta massif was subject to uplifting and erosion, our data shows how in the Upper Oligocene an exhaustion of Cretaceous to Permian sources was followed by an increase in Neo-Proterozoic to Meso-Proterozoic input that is related to the unroofing of the basement rocks, this accelerated exhumation is directly related to the reactivation of the Orihueca Fault as a NW verging thrust at the interior of the massif coeval with Bucaramanga-Santa Marta Fault trans-tensional tectonics in response to the fragmentation of the Farallon plate into the Nazca an Cocos Plates.

  9. Space Weathering of Rocks

    NASA Technical Reports Server (NTRS)

    Noble, Sarah

    2011-01-01

    Space weathering discussions have generally centered around soils but exposed rocks will also incur the effects of weathering. On the Moon, rocks make up only a very small percentage of the exposed surface and areas where rocks are exposed, like central peaks, are often among the least space weathered regions we find in remote sensing data. However, our studies of weathered Ap 17 rocks 76015 and 76237 show that significant amounts of weathering products can build up on rock surfaces. Because rocks have much longer surface lifetimes than an individual soil grain, and thus record a longer history of exposure, we can study these products to gain a deeper perspective on the weathering process and better assess the relative impo!1ance of various weathering components on the Moon. In contrast to the lunar case, on small asteroids, like Itokowa, rocks make up a large fraction of the exposed surface. Results from the Hayabusa spacecraft at Itokowa suggest that while the low gravity does not allow for the development of a mature regolith, weathering patinas can and do develop on rock surfaces, in fact, the rocky surfaces were seen to be darker and appear spectrally more weathered than regions with finer materials. To explore how weathering of asteroidal rocks may differ from lunar, a set of ordinary chondrite meteorites (H, L, and LL) which have been subjected to artificial space weathering by nanopulse laser were examined by TEM. NpFe(sup 0) bearing glasses were ubiquitous in both the naturally-weathered lunar and the artificially-weathered meteorite samples.

  10. Detached rock evaluation device

    DOEpatents

    Hanson, David R.

    1986-01-01

    A rock detachment evaluation device (10) having an energy transducer unit 1) for sensing vibrations imparted to a subject rock (172) for converting the sensed vibrations into electrical signals, a low band pass filter unit (12) for receiving the electrical signal and transmitting only a low frequency segment thereof, a high band pass filter unit (13) for receiving the electrical signals and for transmitting only a high frequency segment thereof, a comparison unit (14) for receiving the low frequency and high frequency signals and for determining the difference in power between the signals, and a display unit (16) for displaying indicia of the difference, which provides a quantitative measure of rock detachment.

  11. Weird 'Endurance' Rock Ahead

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This image taken by the Mars Exploration Rover Opportunity shows a bizarre, lumpy rock dubbed 'Wopmay' on the inner slopes of 'Endurance Crater.' Scientists say the rock's unusual texture is unlike any others observed so far at Meridiani Planum. Wopmay measures approximately 1 meter (3.3 feet) across. The image was taken by the rover's panoramic camera on sol 195 (Aug. 11, 2004). Opportunity will likely travel to this or a similar rock in coming sols for a closer look at the alien surface.

  12. Rock Garden Mosaic

    NASA Technical Reports Server (NTRS)

    1997-01-01

    This image mosaic of part of the 'Rock Garden' was taken by the Sojourner rover's left front camera on Sol 71 (September 14). The rock 'Shark' is at left center and 'Half Dome' is at right. Fine-scale textures on the rocks are clearly seen. Broken crust-like material is visible at bottom center.

    Mars Pathfinder is the second in NASA's Discovery program of low-cost spacecraft with highly focused science goals. The Jet Propulsion Laboratory, Pasadena, CA, developed and manages the Mars Pathfinder mission for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology (Caltech).

  13. Zapping Rocks on Mars

    Wiens, Roger

    2018-01-16

    Better understanding Mars means better understanding its geology. That’s why, sitting atop NASA’s Curiosity rover, is ChemCam, an instrument built by Los Alamos National Laboratory that shoots lasers at Martian rocks and analyzes the data. After nearly 1,500 rock zaps, ChemCam has uncovered some surprising facts about the Red Planet, including the discovery of igneous rocks. Soon, a new Los Alamos-built instrument—the SuperCam—will ride aboard the Mars 2020 rover and bring with it enhanced capabilities to unlock new secrets about the planet.

  14. Zapping Rocks on Mars

    SciT

    Wiens, Roger

    Better understanding Mars means better understanding its geology. That’s why, sitting atop NASA’s Curiosity rover, is ChemCam, an instrument built by Los Alamos National Laboratory that shoots lasers at Martian rocks and analyzes the data. After nearly 1,500 rock zaps, ChemCam has uncovered some surprising facts about the Red Planet, including the discovery of igneous rocks. Soon, a new Los Alamos-built instrument—the SuperCam—will ride aboard the Mars 2020 rover and bring with it enhanced capabilities to unlock new secrets about the planet.

  15. Maximum sedimentation ages and provenance of metasedimentary rocks from Tinos Island, Cycladic blueschist belt, Greece

    NASA Astrophysics Data System (ADS)

    Hinsken, Tim; Bröcker, Michael; Berndt, Jasper; Gärtner, Claudia

    2016-10-01

    U-Pb zircon ages of five metasedimentary rocks from the Lower Unit on Tinos Island (Cycladic blueschist belt, Greece) document supply of detritus from various Proterozoic, Paleozoic and Mesozoic source rocks as well as post-depositional metamorphic zircon formation. Essential features of the studied zircon populations are Late Cretaceous (70-80 Ma) maximum sedimentation ages for the lithostratigraphic succession above the lowermost dolomite marble, significant contributions from Triassic to Neoproterozoic source rocks, minor influx of detritus recording Paleoproterozoic and older provenance (1.9-2.1, 2.4-2.5 and 2.7-2.8 Ga) and a lack or paucity of zircons with Mesoproterozoic ages (1.1-1.8 Ga). In combination with biostratigraphic evidence, the new dataset indicates that Late Cretaceous or younger rocks occur on top of or very close to the basal Triassic metacarbonates, suggesting a gap in the stratigraphic record near the base of the metamorphic succession. The time frame for sediment deposition is bracketed by the youngest detrital zircon ages (70-80 Ma) and metamorphic overgrowths that are related to high-pressure/low-temperature overprinting in the Eocene. This time interval possibly indicates a significant difference to the sedimentation history of the southern Cyclades, where Late Cretaceous detrital zircons have not yet been detected.

  16. Scattering from Rock and Rock Outcrops

    DTIC Science & Technology

    2018-01-23

    scattering and rough areas as seen on the rock outcrop in Fig. 1, display high variability which could pose difficulty for target detection and...classification systems. The primary long-term goal of this research project is to increase understanding and modeling capabilities for high -frequency acoustic...Arlington, VA 22203-1995 10. SPONSOR/MONITOR’S ACRONYM(S) BD025 11. SPONSORING/MONITORING AGENCY REPORT NUMBER 12. DISTRIBUTION AVAILABILITY

  17. Prominent Rocks - 3-D

    1997-07-13

    Many prominent rocks near the Sagan Memorial Station are featured in this image from NASA Mars Pathfinder. Shark, Half-Dome, and Pumpkin are at center 3D glasses are necessary to identify surface detail.

  18. Writing Rock Music Reviews.

    ERIC Educational Resources Information Center

    Brown, Donal

    1980-01-01

    Suggests ways student reviewers of rock music groups can write better reviews. Among the suggestions made are that reviewers occasionally discuss the audience or what makes a particular group unique, support general comment with detail, and avoid ecstatic adjectives. (TJ)

  19. Rock Equity Holdings, LLC

    EPA Pesticide Factsheets

    The EPA is providing notice of an Administrative Penalty Assessment in the form of an Expedited Storm Water Settlement Agreement against Rock Equity Holdings, LLC, for alleged violations at The Cove at Kettlestone/98th Street Reconstruction located at 3015

  20. Early animal evolution: emerging views from comparative biology and geology

    NASA Technical Reports Server (NTRS)

    Knoll, A. H.; Carroll, S. B.

    1999-01-01

    The Cambrian appearance of fossils representing diverse phyla has long inspired hypotheses about possible genetic or environmental catalysts of early animal evolution. Only recently, however, have data begun to emerge that can resolve the sequence of genetic and morphological innovations, environmental events, and ecological interactions that collectively shaped Cambrian evolution. Assembly of the modern genetic tool kit for development and the initial divergence of major animal clades occurred during the Proterozoic Eon. Crown group morphologies diversified in the Cambrian through changes in the genetic regulatory networks that organize animal ontogeny. Cambrian radiation may have been triggered by environmental perturbation near the Proterozoic-Cambrian boundary and subsequently amplified by ecological interactions within reorganized ecosystems.

  1. Mars Rock Analysis Briefing

    2013-03-12

    Paul Mahaffy (right), principal investigator for Curiosity's Sample Analysis at Mars (SAM) investigation at NASA's Goddard Space Flight Center in Maryland, demonstrates how the SAM instrument drilled and captured rock samples on the surface of Mars at a news conference, Tuesday, March 12, 2013 at NASA Headquarters in Washington. The analysis of the rock sample collected shows ancient Mars could have supported living microbes. Photo Credit: (NASA/Carla Cioffi)

  2. Weathering of rock 'Ginger'

    NASA Technical Reports Server (NTRS)

    1997-01-01

    One of the more unusual rocks at the site is Ginger, located southeast of the lander. Parts of it have the reddest color of any material in view, whereas its rounded lobes are gray and relatively unweathered. These color differences are brought out in the inset, enhanced at the upper right. In the false color image at the lower right, the shape of the visible-wavelength spectrum (related to the abundance of weathered ferric iron minerals) is indicated by the hue of the rocks. Blue indicates relatively unweathered rocks. Typical soils and drift, which are heavily weathered, are shown in green and flesh tones. The very red color in the creases in the rock surface correspond to a crust of ferric minerals. The origin of the rock is uncertain; the ferric crust may have grown underneath the rock, or it may cement pebbles together into a conglomerate. Ginger will be a target of future super-resolution studies to better constrain its origin.

    Mars Pathfinder is the second in NASA's Discovery program of low-cost spacecraft with highly focused science goals. The Jet Propulsion Laboratory, Pasadena, CA, developed and manages the Mars Pathfinder mission for NASA's Office of Space Science, Washington, D.C. The Imager for Mars Pathfinder (IMP) was developed by the University of Arizona Lunar and Planetary Laboratory under contract to JPL. Peter Smith is the Principal Investigator. JPL is an operating division of the California Institute of Technology (Caltech).

  3. Comparative geology and geochemistry of sedimentary-rock-hosted (Carlin Type) gold deposits in the People's Republic of China and in Nevada, USA

    Li, Zhiping; Peters, Stephen G.

    1998-01-01

    Sedimentary-rock-hosted (Carlin-type) gold deposits have been considered economically significant and geologically distinct since the early 1960's. This report consists of a nine-part text and an interactive database. This small database is to help Western companies get more information about these gold deposits in China, and to help geologists who are interested in world Carlin-type deposits conduct research on them. Because of their economic significance and geological distinctiveness, these deposits have caught the interest of economic geologists all over the world since the early 1960's. Similar deposits have been discovered in China, Australia, Dominican Republic, Spain, and Russia besides Nevada. Perhaps most significant are the 165 Carlin-type gold deposits that were found in southwest China during the past 15 years. Of these, at least 19 deposits have proven to be of substantial tonnage, making China the second leading country to exploit such deposits. With the increasing interest in Chinese Carlin-type gold deposits, some western companies and geologists desire to get more information about these Chinese deposits. This seems to have been very difficult because the literature was in Chinese. It is estimated that several hundred scientific publications (including papers, books, and technical reports) have been published. This database of Chinese Carlin-type Gold deposits is built on the documentation published during the most recent 10 years and includes six subjects, which consist of 165 records and 30 fields. A new Proterozoic-age sedimentary-rock-hosted gold deposit in northeastern P.R. China also is described. Note that for the old version 1.1 on the CD-ROM, the latitude and longitude locations of the mineral occurrences have been estimated from sketch maps and journal articles and are not intended for digital analysis. One of the improvements in this version 1.2 is the accuracy of geographic data. Version 1.3 updates to the database and includes maps

  4. First U-Pb geochronology on detrital zircons from Early-Middle Cambrian strata of the Torgau-Doberlug Syncline (eastern Germany) and palaeogeographic implications

    NASA Astrophysics Data System (ADS)

    Abubaker, Atnisha; Hofmann, Mandy; Gärtner, Andreas; Linnemann, Ulf; Elicki, Olaf

    2017-10-01

    LA-ICP-MS U-Pb data from detrital zircons of the Ediacaran to Cambrian siliciclastic sequence of the Torgau-Doberlug Syncline (TDS, Saxo-Thuringia, Germany) are reported for the first time. The majority of 203 analysed zircon grains is Proterozoic with minor amount of Archean and Palaeozoic grains. The U-Pb ages fall into three groups: 2.8-2.4 Ga (3%), Neoarchean to earliest Palaeoproterozoic; 2.3-1.6 Ga (46%), early to late Palaeoproterozoic; 1.0-0.5 Ga (47%), Neoproterozoic to Cambrian. This age distribution is typical for the West African Craton as the source area and for Cadomian orogenic events in northwestern Gondwana. The samples show an age gap between 1.6 and 1.0 Ga, which is characteristic for West African provenance and diagnostic in distinguishing this unit from East Avalonia and Baltica. The dataset shows clusters of Palaeoproterozoic ages at 2.2-1.7 Ga, that is typical for western Gondwana, which was affected by abundant magmatic intrusions (ca. 2.2-1.8 Ga) during the Eburnean orogeny (West African craton). Neoarchean zircon ages (3%) point to recycling of magmatic rocks formed during the Liberian and Leonian orogenies. Ediacaran to earliest Cambrian rocks of the TDS originated in an active margin regime of the Gondwanan shelf. The following early Palaeozoic overstep sequence was deposited within rift settings that reflects instability of the West-Gondwanan shelf and the separation of terranes from Ordovician onward. The results of this study demonstrate distinct northwestern African provenance of the Cambrian siliciclastics of the TDS. Due to Th-U ratios from concordant zircon analysis, igneous origin from felsic melts is concluded as the source of these grains.

  5. Distinguishing Grenvillian basement from pre-Taconian cover rocks in the Northern Appalachians

    Karabinos, P.; Aleinikoff, J.N.; Fanning, C.M.

    1999-01-01

    Distinguishing Grenvillian basement rocks from pre-Taconian cover sequences in the Appalachians is a first-order problem essential for accurate structural interpretations. The Cavendish Formation in southeastern Vermont presents a classic example of this problem. Doll and others (1961) showed the Cavendish Formation as younger than the Middle Proterozoic Mount Holly Complex but older than the lithologically similar Cambrian Tyson and Hoosac Formations. More recently, the name Cavendish Formation has been informally abandoned, and its metasedimentary units have been mapped as the Tyson and Hoosac Formations of Late Proterozoic to Cambrian age. In a radical departure from these interpretations, Ratcliffe and others (1997) reassigned metasedimentary rocks of the Cavendish Formation to the Mount Holly Complex based on an inferred intrusive relationship between them and a 1.42 Ga tonalite. This new age assignment, if correct, requires a completely new structural interpretation of the region. SHRIMP and Pb evaporation ages of detrital zircons extracted from a quartzite layer from Cavendish Gorge near the proposed intrusive contact with the tonalite constrain the time of deposition of the Cavendish Formation. Grain shapes of the zircons vary from euhedral to nearly spherical. Virtually all the grains have pitted surfaces and show at least some rounding of edges and terminations; grains exhibit oscillatory zoning typical of zircons that crystallized from a magma. Single-grain Pb evaporation analyses of ten zircons and SHRIMP analyses of 15 zircons all yield ages less than 1.42 Ga. Seven of the grains are consistent with derivation from the Bull Hill Gneiss that postdates the Grenville orogenic cycle and predates deposition of the Cavendish Formation. Thus, the metasedimentary units of the Cavendish Formation should not be assigned to the Mount Holly Complex.

  6. Insights on surface spalling of rock

    NASA Astrophysics Data System (ADS)

    Tarokh, Ali; Kao, Chu-Shu; Fakhimi, Ali; Labuz, Joseph F.

    2016-07-01

    Surface spalling is a complex failure phenomenon that features crack propagation and detachment of thin pieces of rock near free surfaces, particularly in brittle rock around underground excavations when large in situ stresses are involved. A surface instability apparatus was used to study failure of rock close to a free surface, and damage evolution was monitored by digital image correlation (DIC). Lateral displacement at the free face was used as the feedback signal to control the post-peak response of the specimen. DIC was implemented in order to obtain the incremental displacement fields during the spalling process. Displacement fields were computed in the early stage of loading as well as close to the peak stress. Fracture from the spalling phenomenon was revealed by incremental lateral displacement contours. The axial and lateral displacements suggested that the displacement gradient was uniform in both directions at early loading stages and as the load increased, the free-face effect started to influence the displacements, especially the lateral displacement field. A numerical approach, based on the discrete element method, was developed and validated from element testing. Damage evolution and localization observed in numerical simulations were similar to those observed in experiments. By performing simulations in two- and three-dimensions, it was revealed that the intermediate principal stress and platen-rock interfaces have important effects on simulation of surface spalling.

  7. Eos Chaos Rocks

    NASA Technical Reports Server (NTRS)

    2006-01-01

    11 January 2006 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows light-toned, layered rock outcrops in Eos Chaos, located near the east end of the Valles Marineris trough system. The outcrops occur in the form of a distinct, circular butte (upper half of image) and a high slope (lower half of image). The rocks might be sedimentary rocks, similar to those found elsewhere exposed in the Valles Marineris system and the chaotic terrain to the east of the region.

    Location near: 12.9oS, 49.5oW Image width: 3 km (1.9 mi) Illumination from: lower left Season: Southern Summer

  8. Geochemical characteristics of Proterozoic granite magmatism from Southern Granulite Terrain, India: Implications for Gondwana

    NASA Astrophysics Data System (ADS)

    Yellappa, T.; Rao, J. Mallikharjuna

    2018-03-01

    Granitoid intrusions occur widely in the Southern Granulite Terrain (SGT) of India, particularly within the Cauvery Suture Zone (CSZ), which is considered as the trace of the Neoproterozoic Mozambique ocean closure. Here we present the petrological and geochemical features of 19 granite plutons across the three major tectonic blocks of the terrain. Our data show a wide variation in the compositions of these intrusions from alkali feldspathic syenite to granite. The whole rock geochemistry of these intrusions displays higher concentrations of SiO2, FeO*, K2O, Ba, Zr, Th, LREE and low MgO, Na2O, Ti, P, Nb, Y and HREE's. The granitoids are metaluminous to slightly peraluminous in nature revealing both I-type and A-type origin. In tectonic discrimination plots, the plutons dominantly show volcanic arc and syn-collisional as well as post-collisional affinity. Based on the available age data together with geochemical constrains, we demonstrate that the granitic magmatism in the centre and south of the terrain is mostly associated with the Neoproterozoic subduction-collision-accretion-orogeny, followed by extensional mechanism of Gondwana tectonics events. Similar widespread granitic activity has also been documented in the Arabian Nubian shield, Madagascar, Sri Lanka and Antarctica, providing similarities for the reconstruction of the crustal fragments of Gondwana supercontinent followed by Pan-African orogeny.

  9. Identifying early Earth microfossils in unsilicified sediments

    NASA Astrophysics Data System (ADS)

    Javaux, Emmanuelle J.; Asael, Dan; Bekker, Andrey; Debaille, Vinciane; Derenne, Sylvie; Hofmann, Axel; Mattielli, Nadine; Poulton, Simon

    2013-04-01

    The search for life on the early Earth or beyond Earth requires the definition of biosignatures, or "indices of life". These traditionally include fossil molecules, isotopic fractionations, biosedimentary structures and morphological fossils interpreted as remnants of life preserved in rocks. This research focuses on traces of life preserved in unsilicified siliciclastic sediments. Indeed, these deposits preserve well sedimentary structures indicative of past aqueous environments and organic matter, including the original organic walls of microscopic organisms. They also do not form in hydrothermal conditions which may be source of abiotic organics. At our knowledge, the only reported occurrence of microfossils preserved in unsilicified Archean sediments is a population of large organic-walled vesicles discovered in shales and siltstones of the 3.2 Ga Moodies Group, South Africa. (Javaux et al, Nature 2010). These have been interpreted as microfossils based on petrographic and geochemical evidence for their endogenicity and syngeneity, their carbonaceous composition, cellular morphology and ultrastructure, occurrence in populations, taphonomic features of soft wall deformation, and the geological context plausible for life, as well as lack of abiotic explanation falsifying a biological origin. Demonstrating that carbonaceous objects from Archaean rocks are truly old and truly biological is the subject of considerable debate. Abiotic processes are known to produce organics and isotopic signatures similar to life. Spheroidal pseudofossils may form as self-assembling vesicles from abiotic CM, e.g. in prebiotic chemistry experiments (Shoztak et al, 2001), from meteoritic lipids (Deamer et al, 2006), or hydrothermal fluids (Akashi et al, 1996); by artifact of maceration; by migration of abiotic or biotic CM along microfractures (VanZuilen et al, 2007) or along mineral casts (Brasier et al, 2005), or around silica spheres formed in silica-saturated water (Jones and

  10. Soil and Rock Yogi

    1997-07-06

    Several possible targets of study for rover Sojourner's Alpha Proton X-Ray Spectrometer (APXS) instrument are seen in this image, taken by the Imager for Mars Pathfinder (IMP) on Sol 2. The smaller rock at left has been dubbed "Barnacle Bill," while the larger rock at right, approximately 3-4 meters from the lander, is now nicknamed "Yogi." Barnacle Bill is scheduled to be the first object of study for the APXS. Portions of a petal and deflated airbag are also visible at lower right. http://photojournal.jpl.nasa.gov/catalog/PIA00629

  11. Theory of wing rock

    NASA Technical Reports Server (NTRS)

    Hsu, C. H.; Lan, C. E.

    1984-01-01

    A theory is developed for predicting wing rock characteristics. From available data, it can be concluded that wing rock is triggered by flow asymmetries, developed by negative or weakly positive roll damping, and sustained by nonlinear aerodynamic roll damping. A new nonlinear aerodynamic model that includes all essential aerodynamic nonlinearities is developed. The Beecham-Titchener method is applied to obtain approximate analytic solutions for the amplitude and frequency of the limit cycle based on the three degree-of-freedom equations of motion. An iterative scheme is developed to calculate the average aerodynamic derivatives and dynamic characteristics at limit cycle conditions. Good agreement between theoretical and experimental results is obtained.

  12. Re-Os ages for Archean molybdenite and pyrite, Kuittila-Kivisuo, Finland and Proterozoic molybdenite, Kabeliai, Lithuania: Testing the chronometer in a metamorphic and metasomatic setting

    Stein, H.J.; Sundblad, K.; Markey, R.J.; Morgan, J.W.; Motuza, G.

    1998-01-01

    Seven 187Re-187Os ages were determined for molybdenite and pyrite samples from two well-dated Precambrian intrusions in Fennoscandia to examine the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new 187Re decay constant (1.666 x 10-11y-1) with a much improved uncertainty (±0.31%), we determined replicate Re-Os ages for molybdenite and pyrite from the Kuittila and Kivisuo prospects in easternmost Finland and for molybdenite from the Kabeliai prospect in southernmost Lithuania. These two localities contain some of the oldest and youngest plutonic activity in Fennoscandia and are associated with newly discovered economic Au mineralization (Ilomantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-Os ages for veinhosted Kabeliai molybdenite average 1486 ± 5 Ma, in excellent agreement with a 1505 ± 11 Ma U-Pb zircon age for the hosting Kabeliai granite pluton. The slightly younger age suggests the introduction of Cu-Mo mineralization by a later phase of the Kabeliai magmatic system. Mean Re-Os ages of 2778 ± 8 Ma and 2781 ± 8 Ma for Kuittila and Kivisuo molybdenites, respectively, are in reasonable agreement with a 2753 ± 5 Ma weighted mean U-Pb zircon age for hosting Kuittila tonalite. These Re-Os ages agree well with less precise ages of 2789 ± 290 Ma for a Rb-Sr whole-rock isochron and 2771 ± 75 Ma for the average of six Sm-Nd T(DM) model ages for Kuittila tonalite. Three Re-Os analyses of a single pyrite mineral separate, from the same sample of Kuittila pluton that yielded a molybdenite separate, provide individual model ages of 2710 ± 27, 2777 ± 28, and 2830 ± 28 Ma (Re = 17.4, 12.1, and 8.4 ppb, respectively), with a mean value of 2770 ± 120 Ma in agreement with the Kuittila molybdenite age. The Re and 187Os abundances in these three pyrite splits are highly correlated (r = 0.9994), and provide a 187Re-187Os isochron age of 2607 ± 47 Ma with an intercept of 21 ppt 187Os (MSWD = 1.1). It appears

  13. First Archean Zircons Found in Oceanic Crustal Rocks of Mauritius

    NASA Astrophysics Data System (ADS)

    Ashwal, L. D.; Wiedenbeck, M.; Torsvik, T. H.

    2016-12-01

    A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius, on both the basis of inversion of gravity anomaly data (crustal thickness) and the recovery of Proterozoic zircons (660-1971 Ma) from basaltic beach sands (Torsvik et al., Nature Geosci. 6, 227, 2013). We recovered 13 zircon grains from a trachyte associated with the Older Series basalts (9.0-4.7 Ma) of Mauritius, the second youngest member of a hot-spot track extending from the active plume site of Réunion. Extreme care was taken to avoid contamination during sample processing. Ten of the 13 grains are featureless, with no internal structures, and SIMS analyses (Cameca 1280-HR instrument) yield 49 spots with Miocene U-Pb systematics and a mean age of 5.7 ± 0.2 Ma (1 sd), constraining the magmatic crystallization age of the trachyte. Three grains with partially resorbed magmatic zoning, partial metamictization and mineral inclusions (quartz, K-feldspar, monazite) show uniquely mid- to late-Archean systematics: 20 spot analyses give concordant to near-concordant ages of 3030 ± 5 Ma to 2766 ± 13 Ma. This suggests that during ascent, the trachytic magmas incorporated silicic continental crustal material that preserves a record of several hundred m.y. of Archean evolution. This is consistent with Sr-Nd isotope systematics of the Mauritian trachytes, which can be modelled as having been contaminated with 0.4-3.5% of ancient granitoid crustal components. Our new age results, combined with the Proterozoic ages of zircons recovered from Mauritian beach sands, are best correlated with continental crust of east-central Madagascar, presently 700 km west of Mauritius, where Archean gneisses and Neoproterozoic intrusive rocks are juxtaposed such that a 2000 km2 area could correspond to a fragment of continent presently underlying Mauritius. This, and other continental fragments formed during Gondwana break-up, may be scattered across the

  14. Paleobiological Perspectives on Early Microbial Evolution

    PubMed Central

    Knoll, Andrew H.

    2015-01-01

    Microfossils, stromatolites, and chemical biosignatures indicate that Earth became a biological planet more than 3.5 billion years ago, making most of life's history microbial. Proterozoic rocks preserve a rich record of cyanobacteria, including derived forms that differentiate multiple cell types. Stromatolites, in turn, show that microbial communities covered the seafloor from tidal flats to the base of the photic zone. The Archean record is more challenging to interpret, particularly on the question of cyanobacterial antiquity, which remains to be resolved. In the late Neoproterozoic Era, increasing oxygen and radiating eukaryotes altered the biosphere, with planktonic algae gaining ecological prominence in the water column, whereas seaweeds and, eventually, animals spread across shallow seafloors. From a microbial perspective, however, animals, algae, and, later, plants simply provided new opportunities for diversification, and, to this day, microbial metabolisms remain the only essential components of biogeochemical cycles. PMID:26134315

  15. Proterozoic geochronological links between the Farewell, Kilbuck, and Arctic Alaska terranes

    Bradley, Dwight C.; McClelland, William C.; Friedman, Richard M.; O'Sullivan, Paul B.; Layer, Paul; Miller, Marti L.; Dumoulin, Julie A.; Till, Alison B.; Abbott, J. Grant; Bradley, Dan B.; Wooden, Joseph L.

    2014-01-01

    New U-Pb igneous and detrital zircon ages reveal that despite being separated by younger orogens, three of Alaska’s terranes that contain Precambrian rocks—Farewell, Kilbuck, and Arctic Alaska—are related. The Farewell and Kilbuck terranes can be linked by felsic magmatism at ca. 850 Ma and by abundant detrital zircons in the Farewell that overlap the ca. 2010–2085 Ma age range of granitoids in the Kilbuck. The Farewell and Arctic Alaska terranes have already been linked via correlative Neoproterozoic to Devonian carbonate platform deposits that share nearly identical faunas of mixed Siberian and Laurentian affinity. New igneous ages strengthen these ties. Specifically, 988, 979, and 979 Ma metafelsites in the Farewell terrane are close in age to a 971 Ma granitic orthogneiss in the Arctic Alaska terrane. Likewise, 852, 850, 845, and 837 Ma granitic orthogneisses, metafelsite, and rhyolite in the Farewell terrane are similar to the reported 874 to 848 Ma age range of metarhyolites in the Arctic Alaska terrane. The Kilbuck and Arctic Alaska terranes have been previously linked on the basis of provenance: detrital zircons from the Carboniferous Nuka Formation in the Arctic Alaska terrane range from 2013 to 2078 Ma, overlapping the age of Kilbuck granitoids. A new 849 Ma age of a Kilbuck granitoid strengthens the proposed connection. Among the other new results from Kilbuck terrane is a 2085 Ma zircon from a granitoid that now stands as the oldest tightly dated rock in Alaska. We conclude that the Kilbuck, Farewell, and Arctic Alaska terranes were not independent entities with unique geologic histories but instead are related pieces of the circum-Arctic tectonic puzzle.

  16. Northeast Church Rock Mine

    EPA Pesticide Factsheets

    Northeast Church Rock Mine, a former uranium mine 17 miles northeast of Gallup, NM in the Pinedale Chapter of the Navajo Nation. EPA is working with NNEPA to oversee cleanup work by United Nuclear Corporation, a company owned by General Electric (GE).

  17. Wind Carved Rock

    2016-10-19

    The distinctively fluted surface and elongated hills in this image in Medusae Fossae are caused by wind erosion of a soft fine-grained rock. Called yardangs, these features are aligned with the prevailing wind direction. This wind direction would have dominated for a very long time to carve these large-scale features into the exposed rock we see today. Yardangs not only reveal the strength and direction of historic winds, but also reveal something of the host rock itself. Close inspection by HiRISE shows an absence of boulders or rubble, especially along steep yardang cliffs and buttresses. The absence of rubble and the scale of the yardangs tells us that the host rock consists only of weakly cemented fine granules in tens of meters or more thick deposits. Such deposits could have come from extended settling of volcanic ash, atmospheric dust, or accumulations of wind deposited fine sands. After a time these deposits became cemented and cohesive, illustrated by the high standing relief and exposed cliffs. http://photojournal.jpl.nasa.gov/catalog/PIA21111

  18. Slippery Rock University

    ERIC Educational Resources Information Center

    Arnhold, Robert W.

    2008-01-01

    Slippery Rock University (SRU), located in western Pennsylvania, is one of 14 state-owned institutions of higher education in Pennsylvania. The university has a rich tradition of providing professional preparation programs in special education, therapeutic recreation, physical education, and physical therapy for individuals with disabilities.…

  19. A Rock Retrospective.

    ERIC Educational Resources Information Center

    O'Grady, Terence J.

    1979-01-01

    The author offers an analysis of musical techniques found in the major rock trends of the 1960s. An annotated list of selected readings and a subject-indexed list of selected recordings are appended. This article is part of a theme issue on popular music. (Editor/SJL)

  20. Curiosity First Rock Star

    2012-08-17

    This mosaic image shows the first target NASA Curiosity rover aims to zap ChemCam instrument. ChemCam will be firing a laser at this rock, provisionally named N165, and analyzing the glowing, ionized gas, called plasma, that the laser excites.

  1. Teaching the Rock Cycle with Ease.

    ERIC Educational Resources Information Center

    Bereki, Debra

    2000-01-01

    Describes a hands-on lesson for teaching high school students the concept of the rock cycle using sedimentary, metamorphic, and igneous rocks. Students use a rock cycle diagram to identify pairs of rocks. From the rock cycle, students explain on paper how their first rock became the second rock and vice versa. (PVD)

  2. Proterozoic to Mesozoic evolution of North-West Africa and Peri-Gondwana microplates: Detrital zircon ages from Morocco and Canada

    NASA Astrophysics Data System (ADS)

    Marzoli, Andrea; Davies, Joshua H. F. L.; Youbi, Nasrrddine; Merle, Renaud; Dal Corso, Jacopo; Dunkley, Daniel J.; Fioretti, Anna Maria; Bellieni, Giuliano; Medina, Fida; Wotzlaw, Jörn-Frederik; McHone, Greg; Font, Eric; Bensalah, Mohamed Khalil

    2017-05-01

    The complex history of assemblage and disruption of continental plates surrounding the Atlantic Ocean is in part recorded by the distribution of detrital zircon ages entrained in continental sedimentary strata from Morocco (Central High Atlas and Argana basins) and Canada (Grand Manan Island, New Brunswick). Here we investigate detrital zircon from the latest Triassic (ca. 202 Ma) sedimentary strata directly underlying lava flows of the Central Atlantic magmatic province or interlayered within them. SHRIMP (Sensitive High-Resolution Ion MicroProbe) and LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry) U-Pb ages for zircon range from Paleozoic to Archean with a dominant Neoproterozoic peak, and significant amounts of ca. 2 Ga zircon. These ages suggest a prevailing West African (Gondwanan) provenance at all sampling sites. Notably, the Paleoproterozoic zircon population is particularly abundant in central Morocco, north of the High Atlas chain, suggesting the presence of Eburnean-aged rocks in this part of the country, which is consistent with recent geochronologic data from outcropping rocks. Minor amounts of late Mesoproterozoic and early Neoproterozoic zircon ages (ca. 1.1-0.9 Ga) in Moroccan samples are more difficult to interpret. A provenance from Avalonia or Amazonia, as proposed by previous studies is not supported by the age distributions observed here. An involvement of more distal source regions, possibly located in north-eastern Africa (Arabian Nubian Shield) would instead be possible. Paleozoic zircon ages are abundant in the Canadian sample, pointing to a significant contribution from Hercynian aged source rocks. Such a signal is nearly absent in the Moroccan samples, suggesting that zircon-bearing Hercynian granitic rocks of the Moroccan Meseta block were not yet outcropping at ca. 200 Ma. The only Moroccan samples that yield Paleozoic zircon ages are those interlayered within the CAMP lavas, suggesting an increased dismantling

  3. Elastic Rock Heterogeneity Controls Brittle Rock Failure during Hydraulic Fracturing

    NASA Astrophysics Data System (ADS)

    Langenbruch, C.; Shapiro, S. A.

    2014-12-01

    For interpretation and inversion of microseismic data it is important to understand, which properties of the reservoir rock control the occurrence probability of brittle rock failure and associated seismicity during hydraulic stimulation. This is especially important, when inverting for key properties like permeability and fracture conductivity. Although it became accepted that seismic events are triggered by fluid flow and the resulting perturbation of the stress field in the reservoir rock, the magnitude of stress perturbations, capable of triggering failure in rocks, can be highly variable. The controlling physical mechanism of this variability is still under discussion. We compare the occurrence of microseismic events at the Cotton Valley gas field to elastic rock heterogeneity, obtained from measurements along the treatment wells. The heterogeneity is characterized by scale invariant fluctuations of elastic properties. We observe that the elastic heterogeneity of the rock formation controls the occurrence of brittle failure. In particular, we find that the density of events is increasing with the Brittleness Index (BI) of the rock, which is defined as a combination of Young's modulus and Poisson's ratio. We evaluate the physical meaning of the BI. By applying geomechanical investigations we characterize the influence of fluctuating elastic properties in rocks on the probability of brittle rock failure. Our analysis is based on the computation of stress fluctuations caused by elastic heterogeneity of rocks. We find that elastic rock heterogeneity causes stress fluctuations of significant magnitude. Moreover, the stress changes necessary to open and reactivate fractures in rocks are strongly related to fluctuations of elastic moduli. Our analysis gives a physical explanation to the observed relation between elastic heterogeneity of the rock formation and the occurrence of brittle failure during hydraulic reservoir stimulations. A crucial factor for understanding

  4. Zircon Lu-Hf isotope systematics and U-Pb geochronology, whole-rock Sr-Nd isotopes and geochemistry of the early Jurassic Gokcedere pluton, Sakarya Zone-NE Turkey: a magmatic response to roll-back of the Paleo-Tethyan oceanic lithosphere

    NASA Astrophysics Data System (ADS)

    Karsli, Orhan; Dokuz, Abdurrahman; Kandemir, Raif

    2017-05-01

    The early Mesozoic was a critical era for the geodynamic evolution of the Sakarya Zone as transition from accretion to collision events in the region. However, its complex evolutionary history is still debated. To address this issue, we present new in situ zircon U-Pb ages and Lu-Hf isotope data, whole-rock Sr-Nd isotopes, and mineral chemistry and geochemistry data of plutonic rocks to better understand the magmatic processes. The Gokcedere pluton is mainly composed of gabbro and gabbroic diorite. LA-ICP-MS zircon U-Pb dating reveals that the pluton was emplaced in the early Jurassic (177 Ma). These gabbros and gabbroic diorites are characterized by relatively low SiO2 content of 47.09 to 57.15 wt% and high Mg# values varying from 46 to 75. The samples belong to the calc-alkaline series and exhibit a metaluminous I-type character. Moreover, they are slightly enriched in large ion lithophile elements (Rb, Ba, Th and K) and light rare earth elements and depleted in high field strength elements (Nb and Ti). Gabbroic rocks of the pluton have a depleted Sr-Nd isotopic composition, including low initial 87Sr/86Sr ranging from 0.705124 to 0.705599, relatively high ɛ Nd ( t) values varying from 0.1 to 3.5 and single-stage Nd model ages ( T DM1 = 0.65-0.95 Ga). In situ zircon analyses show that the rocks have variable and positive ɛ Hf ( t) values (4.6 to 13.5) and single-stage Hf model ages ( T DM1 = 0.30 to 0.65 Ga). Both the geochemical signature and Sr-Nd-Hf isotopic composition of the gabbroic rocks reveal that the magma of the studied rocks was formed by the partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The influence of slab fluids is mirrored by their trace-element characteristics. Trace-element modeling suggests that the primary magma was generated by a low and variable degree of partial melting ( 5-15%) of a depleted and young lithospheric mantle wedge consisting of phlogopite- and spinel-bearing lherzolite. Heat to melt the

  5. Metasomatic alkali-feldspar syenites (episyenites) of the Proterozoic Suomenniemi rapakivi granite complex, southeastern Finland

    NASA Astrophysics Data System (ADS)

    Suikkanen, E.; Rämö, O. T.

    2017-12-01

    Peralkaline to marginally metaluminous alkali-feldspar syenites and quartz alkali-feldspar syenites are hosted by subalkaline, ferroan rapakivi granites in the 1644 Ma Suomenniemi complex of southeastern Finland. These alkali syenites form NW-oriented dikes and small (< 10 m in diameter) bodies that are distinguished from the surrounding granites by their color (violet-red), general lack of quartz, as well as pronounced interstitial character of mafic minerals. Microtextures of the syenites imply pervasive alkali metasomatism and growth of secondary sodic and oxidized ferromagnesian minerals. Both subsolvus ( Ab99 and Or90-100Ab0-10) and hypersolvus (Or40-60Ab40-60) feldspar assemblages are present and display red luminescence characteristic of alkali feldspar recrystallized in the presence of an oxidizing fluid. In the marginally metaluminous syenites, primary magmatic hastingsite has been metasomatized to ferro-actinolite or decomposed to ferro-ferri-hornblende and magnetite. In some of the peralkaline syenites, primary hastingsite was replaced by magnetite and feldspars and has been overgrown by aegirine-augite and riebeckite. Sodic clinopyroxene (sodic augite-aegirine) is the most common and, in many cases, the only ferromagnesian silicate in these syenites. Three peralkaline alkali-feldspar syenites analyzed for zircon U-Pb and O isotopic compositions by single-grain SIMS have zircon 207Pb/206Pb ages of 1645 ± 5, 1642 ± 4 and 1644 ± 4 Ma, and zircon δ18OVSMOW values of 8.04 ± 0.18, 8.19 ± 0.17 and 8.26 ± 0.17‰. Whole-rock Nd isotope data imply an overall εNd(1644 Ma) value of ca. - 1.5 for the syenites. These ages and isotopic fingerprints are, within error, identical to those of the subalkaline granites of the complex. We propose that the Suomenniemi alkali-feldspar syenites are episyenites, formed as the result of pervasive local metasomatism of the subalkaline granites caused by high-temperature oxidizing peralkaline fluids. The process led to

  6. Detrital zircon ages from southern Norway - implications for the Proterozoic evolution of the southwestern Baltic Shield

    NASA Astrophysics Data System (ADS)

    Knudsen, T.-L.; Andersen, T.; Whitehouse, M. J.; Vestin, J.

    An ion-microprobe (SIMS) U-Pb zircon dating study on four samples of Precambrian metasediments from the high-grade Bamble Sector, southern Norway, gives the first information on the timing of discrete crust-forming events in the SW part of the Baltic Shield. Recent Nd and Pb studies have indicated that the sources of the clastic metasediments in this area have crustal histories extending back to 1.7 to 2.1Ga, although there is no record of rocks older than 1.6Ga in southern Norway. The analysed metasediments are from a sequence of intercalated, centimetre to 10-metre wide units of quartzites, semi-metapelites, metapelites and mafic granulites. The zircons can be grouped in two morphological populations: (1) long prismatic; (2) rounded, often flattened. The BSE images reveal that both populations consist of oscillatory zoned, rounded and corroded cores (detrital grains of magmatic origin), surrounded by homogeneous rims (metamorphic overgrowths). The detrital zircons have 207Pb/206Pb ages between 1367 and 1939Ma, with frequency maxima in the range 1.85 to 1.70Ga and 1.60 to 1.50Ga. There is no correlation between crystal habit and age of the zircon. One resorbed, inner zircon core in a detrital grain is strongly discordant and gives a composite inner core-magmatic outer core 207Pb/206Pb age of 2383 Ma. Two discrete, unzoned zircons have 207Pb/206Pb ages of 1122 and 1133Ma, representing zircon growth during the Sveconorwegian high-grade metamorphism. Also the μm wide overgrowths, embayments in the detrital cores and apparent ``inner cores'' which represent secondary metamorphic zircon growth in deep embayments in detrital grains, are of Sveconorwegian age. The composite-detrital-metamorphic zircon analyses give generally discordant 206Pb/238U versus 207Pb/235U ratios and maximum 207Pb/206Pb ages of 1438Ma. These data demonstrate the existence of a protocrust of 1.7 to 2.0Ga in the southwestern part of the Baltic Shield, implying a break in the overall westward

  7. Martian Rocks Rich in Silicon

    2014-09-11

    Data from the Alpha Particle X-ray Spectrometer APXS instrument on NASA Mars rover Curiosity show an unusual enrichment of silicon in the rocks dubbed Wildrose and Bonanza King, relative to other rocks studied at Gale Crater on Mars.

  8. [Rock music and hearing disorders].

    PubMed

    Størmer, Carl Christian Lein; Stenklev, Niels Christian

    2007-03-29

    Continued exposition to loud noise is a well-known risk factor for development of various hearing disorders; rock musicians are especially vulnerable. The aim of this paper was to get an overview of hearing loss, tinnitus and hyperacusis among rock musicians. Medline was systematically searched, using combinations of the terms "hearing", "rock music", "tinnitus" and "hyperacusis". Seven publications concerning hearing of rock musicians were identified. Permanent hearing loss occurred in 20% (mean) of the rock musicians; the prevalence varied from 5 to 41%. Tinnitus and hyperacusis appear significantly more often in rock musicians than in non-musicians. Rock musicians have increased resistance against loud music and exposure over time is protective towards hearing loss. Further research is needed to assess rock music's impact on musicians' hearing.

  9. Grenville age of basement rocks in Cape May NJ well: New evidence for Laurentian crust in U.S. Atlantic Coastal Plain basement Chesapeake terrane

    Sheridan, R.E.; Maguire, T.J.; Feigenson, M.D.; Patino, L.C.; Volkert, R.A.

    1999-01-01

    The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/Strontium dating of the Chesapeake terrane basement yields an age of 1.025 ?? 0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO2 and Zr/P2O5 composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment.

  10. Early and middle(?) Cambrian metazoan and protistan fossils from West Africa

    Culver, S.J.; Repetski, J.E.; Pojeta, J.; Hunt, D.

    1996-01-01

    Supposed Upper Proterozoic strata in the southwest Taoudeni Basin, Guinea and Senegal, and from the Mauritanide fold belt, Mauritania, have yielded mostly poorly preserved small skeletal fossils of metazoan and protistan origin. Problematic, but possible echinoderm material and spicules of the heteractinid sponge Eiffelia dominate the Taoudeni Basin assemblage. The age of the material is not certain but the paleontologic data suggest an Early Cambrian age for the stratigraphically lowest faunas, and a Middle Cambrian age is possible for the stratigraphically highest collections.

  11. Microwave assisted hard rock cutting

    DOEpatents

    Lindroth, David P.; Morrell, Roger J.; Blair, James R.

    1991-01-01

    An apparatus for the sequential fracturing and cutting of subsurface volume of hard rock (102) in the strata (101) of a mining environment (100) by subjecting the volume of rock to a beam (25) of microwave energy to fracture the subsurface volume of rock by differential expansion; and , then bringing the cutting edge (52) of a piece of conventional mining machinery (50) into contact with the fractured rock (102).

  12. Lead-alpha age determinations of granitic rocks from Alaska

    Matzko, John J.; Jaffe, H.W.; Waring, C.L.

    1957-01-01

    Lead-alpha activity age determinations were made on zircon from seven granitic rocks of central and southeastern Alaska. The results of the age determinations indicate two periods of igneous intrusion, one about 95 million years ago, during the Cretaceous period, and another about 53 million years ago, during the early part of the Tertiary. The individual ages determined on zircon from 2 rocks from southeastern Alaska and 1 from east-central Alaska gave results of 90, 100, and 96 million years; those determined on 4 rocks from central Alaska gave results of 47, 56, 58, and 51 million years.

  13. Realistic Expectations for Rock Identification.

    ERIC Educational Resources Information Center

    Westerback, Mary Elizabeth; Azer, Nazmy

    1991-01-01

    Presents a rock classification scheme for use by beginning students. The scheme is based on rock textures (glassy, crystalline, clastic, and organic framework) and observable structures (vesicles and graded bedding). Discusses problems in other rock classification schemes which may produce confusion, misidentification, and anxiety. (10 references)…

  14. Rock burst governance of working face under igneous rock

    NASA Astrophysics Data System (ADS)

    Chang, Zhenxing; Yu, Yue

    2017-01-01

    As a typical failure phenomenon, rock burst occurs in many mines. It can not only cause the working face to cease production, but also cause serious damage to production equipment, and even result in casualties. To explore how to govern rock burst of working face under igneous rock, the 10416 working face in some mine is taken as engineering background. The supports damaged extensively and rock burst took place when the working face advanced. This paper establishes the mechanical model and conducts theoretical analysis and calculation to predict the fracture and migration mechanism and energy release of the thick hard igneous rock above the working face, and to obtain the advancing distance of the working face when the igneous rock fractures and critical value of the energy when rock burst occurs. Based on the specific conditions of the mine, this paper put forward three kinds of governance measures, which are borehole pressure relief, coal seam water injection and blasting pressure relief.

  15. Moon rock in JPM

    2009-06-07

    ISS020-E-007383 (FOR RELEASE 21 JULY 2009) --- A moon rock brought to Earth by Apollo 11, humans? first landing on the moon in July 1969, is shown as it floats aboard the International Space Station. Part of Earth and a section of a station solar panel can be seen through the window. The 3.6 billion year-old lunar sample was flown to the station aboard Space Shuttle mission STS-119 in April 2009 in honor of the July 2009 40th anniversary of the historic first moon landing. The rock, lunar sample 10072, was flown to the station to serve as a symbol of the nation?s resolve to continue the exploration of space. It will be returned on shuttle mission STS-128 to be publicly displayed.

  16. Unboxing Space Rocks

    Bruck Syal, Megan

    2018-01-16

    The box was inconspicuous, but Lawrence Livermore National Laboratory (LLNL) postdoctoral researcher Megan Bruck Syal immediately knew its contents: two meteorites around the size of walnuts. They formed about 4.6 billion years ago and survived a history of violent collisions in the asteroid belt before being bumped into a near-Earth-object orbit by gravitational interactions with the planets. After finally raining down on Earth, these rocks were scavenged in Antarctica by researchers, sorted and classified at NASA Johnson Space Center, then mailed first-class to Bruck Syal. Now that these space rocks are in Bruck Syal’s hands, they are mere months away from fulfilling their destiny. They are to be vaporized by a high-powered laser, and the data they yield on asteroid deflection could one day save the planet.

  17. Unboxing Space Rocks

    SciT

    Bruck Syal, Megan

    The box was inconspicuous, but Lawrence Livermore National Laboratory (LLNL) postdoctoral researcher Megan Bruck Syal immediately knew its contents: two meteorites around the size of walnuts. They formed about 4.6 billion years ago and survived a history of violent collisions in the asteroid belt before being bumped into a near-Earth-object orbit by gravitational interactions with the planets. After finally raining down on Earth, these rocks were scavenged in Antarctica by researchers, sorted and classified at NASA Johnson Space Center, then mailed first-class to Bruck Syal. Now that these space rocks are in Bruck Syal’s hands, they are mere months awaymore » from fulfilling their destiny. They are to be vaporized by a high-powered laser, and the data they yield on asteroid deflection could one day save the planet.« less

  18. Terrain and Rock Yogi

    1997-07-06

    The left portion of this image, taken by the Imager for Mars Pathfinder (IMP) on July 5, 1997 (Sol 2), shows a portion of the large rock nicknamed "Yogi." Portions of a petal and deflated airbag are in the foreground. The dark circular object at right is a portion of the lander's high-gain antenna. http://photojournal.jpl.nasa.gov/catalog/PIA00630

  19. Soil Rock Analyzer

    NASA Technical Reports Server (NTRS)

    1985-01-01

    A redesigned version of a soil/rock analyzer developed by Martin Marietta under a Langley Research Center contract is being marketed by Aurora Tech, Inc. Known as the Aurora ATX-100, it has self-contained power, an oscilloscope, a liquid crystal readout, and a multichannel spectrum analyzer. It measures energy emissions to determine what elements in what percentages a sample contains. It is lightweight and may be used for mineral exploration, pollution monitoring, etc.

  20. Probabilistic Rock Slope Engineering.

    DTIC Science & Technology

    1984-06-01

    4 U rmy Corps PROBABILISTIC ROCK SLOPE ENGINEERING by Stanley M. Miller jGeotechnical Engineer 509 E. Calle Avenue Tucson, Arizona 85705 Co N 00 IFI...NUMBERS Geological Engineer CW71 1ork Unit 31755 509 E. Calle Avenue, Tucson, Arizona 85705 11. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE...communication, J. P. Sa,.-1Iy, Inspiration Consolidated Copper Co., Inspiration, Ariz., 1980. Personal communication, R. D. Call, Pincock, Allen, and

  1. Mars Rock Analysis Briefing

    2013-03-12

    Paul Mahaffy, principal investigator for Curiosity's Sample Analysis at Mars (SAM) investigation at NASA's Goddard Space Flight Center in Maryland, answer's a reporters question at a news conference, Tuesday, March 12, 2013 at NASA Headquarters in Washington. The news conference covered the findings that the analysis of the rock sample collected shows ancient Mars could have supported living microbes. Photo Credit: (NASA/Carla Cioffi)

  2. Ripples and Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    26 February 2005 This Mars Global Surveyor (MGS) Orbiter Camera (MOC) image shows light-toned sedimentary rock outcrops and large dark-toned, windblown ripples in Aram Chaos.

    Location near: 3.0oN, 20.9oW Image width: 3 km (1.9 mi) Illumination from: upper left Season: Northern Summer

  3. Rock pushing and sampling under rocks on Mars

    Moore, H.J.; Liebes, S.; Crouch, D.S.; Clark, L.V.

    1978-01-01

    Viking Lander 2 acquired samples on Mars from beneath two rocks, where living organisms and organic molecules would be protected from ultraviolet radiation. Selection of rocks to be moved was based on scientific and engineering considerations, including rock size, rock shape, burial depth, and location in a sample field. Rock locations and topography were established using the computerized interactive video-stereophotogrammetric system and plotted on vertical profiles and in plan view. Sampler commands were developed and tested on Earth using a full-size lander and surface mock-up. The use of power by the sampler motor correlates with rock movements, which were by plowing, skidding, and rolling. Provenance of the samples was determined by measurements and interpretation of pictures and positions of the sampler arm. Analytical results demonstrate that the samples were, in fact, from beneath the rocks. Results from the Gas Chromatograph-Mass Spectrometer of the Molecular Analysis experiment and the Gas Exchange instrument of the Biology experiment indicate that more adsorbed(?) water occurs in samples under rocks than in samples exposed to the sun. This is consistent with terrestrial arid environments, where more moisture occurs in near-surface soil un- der rocks than in surrounding soil because the net heat flow is toward the soil beneath the rock and the rock cap inhibits evaporation. Inorganic analyses show that samples of soil from under the rocks have significantly less iron than soil exposed to the sun. The scientific significance of analyses of samples under the rocks is only partly evaluated, but some facts are clear. Detectable quantities of martian organic molecules were not found in the sample from under a rock by the Molecular Analysis experiment. The Biology experiments did not find definitive evidence for Earth-like living organisms in their sample. Significant amounts of adsorbed water may be present in the martian regolith. The response of the soil

  4. Major structural controls on the distribution of pre-Tertiary rocks, Nevada Test Site vicinity, southern Nevada

    Cole, James C.

    1997-01-01

    The lateral and vertical distributions of Proterozoic and Paleozoic sedimentary rocks in southern Nevada are the combined products of original stratigraphic relationships and post-depositional faults and folds. This map compilation shows the distribution of these pre-Tertiary rocks in the region including and surrounding the Nevada Test Site. It is based on considerable new evidence from detailed geologic mapping, biostratigraphic control, sedimentological analysis, and a review of regional map relationships.Proterozoic and Paleozoic rocks of the region record paleogeographic transitions between continental shelf depositional environments on the east and deeper-water slopefacies depositional environments on the west. Middle Devonian and Mississippian sequences, in particular, show strong lateral facies variations caused by contemporaneous changes in the western margin of North America during the Antler orogeny. Sections of rock that were originally deposited in widely separated facies localities presently lie in close proximity. These spatial relationships chiefly result from major east- and southeastdirected thrusts that deformed the region in Permian or later time.Somewhat younger contractional structures are identified within two irregular zones that traverse the region. These folds and thrusts typically verge toward the west and northwest and overprint the relatively simple pattern of the older contractional terranes. Local structural complications are significant near these younger structures due to the opposing vergence and due to irregularities in the previously folded and faulted crustal section.Structural and stratigraphic discontinuities are identified on opposing sides of two north-trending fault zones in the central part of the compilation region north of Yucca Flat. The origin and significance of these zones are enigmatic because they are largely covered by Tertiary and younger deposits. These faults most likely result from significant lateral offset

  5. Chemical Mapping of Proterozoic Organic Matter at Sub-Micron Spatial Resolution

    NASA Technical Reports Server (NTRS)

    Oehler, Dorothy Z.; Robert, Francois; Mostefaoui, Smail; Meibom, Anders; Selo, Madeleine; McKay, David S.

    2006-01-01

    We have used a NanoSIMS ion microprobe to map sub-micron-scale distributions of carbon, nitrogen, sulfur, silicon, and oxygen in organic microfossils and laminae from the approximately 0.85 Ga Bitter Springs Formation of Australia. The data provide clues about the original chemistry of the microfossils, the silicification process, and biosignatures of specific microorganisms and microbial communities. Chemical maps of fossil unicells and filaments reveal distinct wall-and sheath-like structures enriched in C, N and S, consistent with their accepted biological origin. Surprisingly, organic laminae, previously considered to be amorphous, also exhibit filamentous and apparently compressed spheroidal structures defined by strong enrichments in C, N and S. By analogy to data from the well-preserved microfossils, these structures are interpreted as being of biological origin, most likely representing densely packed remnants of microbial mats. Because the preponderance of organic matter in Precambrian sediments is similarly "amorphous," our findings open a large body of generally neglected material to in situ structural, chemical, and isotopic study. Our results also offer new criteria for assessing biogenicity of problematic kerogenous materials and thus can be applied to assessments of poorly preserved or fragmentary organic residues in early Archean sediments and any that might occur in meteorites or other extraterrestrial samples.

  6. Sedimentary rocks of the coast of Liberia

    White, Richard William

    1969-01-01

    Two basins containing sedimentary rocks o# probable Cretaceous age have been recognized near the coast of Liberia in the area between Monrovia and Buchanan; geophysical evidence suggests that similar though larger basins exist on the adjacent continental shelf. The oldest sedimentary unit recognized, the Paynesville Sandstone of possible early to middle Paleozoic age, is intruded by dikes and sills of diabase of early Jurassic age and lies unconformably on crystalline rocks of late Precambrian age. Dips in the Paynesville Sandstone define a structural basin centered south of Roberts International Airport (formerly called Roberts Field) about 25 miles east of Monrovla. Wackes and conglomerates of Cretaceous age, herein named the Farmington River Formation, unconformably overlie the Paynesville Sandstone and constitute the sedimentary fill in the Roberts basin. The Bassa basin lies to the southeast of the Roberts basin and is separated from it by an upwarp of crystalline rocks. The basin is occupied by wackes and conglomerates of the Farmington River Formation, which apparently lie directly on the crystalline basement. Both basins are bounded on the northeast by northwest-trending dip-slip faults. The best potential for petroleum deposits that exists in Liberia is beneath the adjacent continental shelf and slope. Geophysical exploration and drilling will be required to evaluate this potential.

  7. Co-rich sulfides in mantle peridotites from Penghu Islands, Taiwan: Footprints of Proterozoic mantle plumes under the Cathaysia Block

    NASA Astrophysics Data System (ADS)

    Wang, Kuo-Lung; O'Reilly, Suzanne Y.; Honda, Masahiko; Matsumoto, Takuya; Griffin, William L.; Pearson, Norman J.; Zhang, Ming

    2010-02-01

    Proterozoic evolution ( Li et al. (2008) Precambrian Research 160, 179-210 and references therein). Olivine in a peridotite sample from the TCY locality has distinctly high 3He/ 4He (11 R A), whereas other peridotites from the KP and TCY localities have 3He/ 4He ˜6.7 R A, lower than MORB. The high 3He/ 4He further suggests that materials from the deep mantle have interacted with the host peridotite of Co-rich sulfides. We thus propose that the Co-rich sulfide melts may have been trapped in the lower mantle during core-mantle differentiation and then transported to shallow depths by mantle plumes that entrained lower mantle materials at several different time periods. This study provides the first substantial evidence from the lithosperic mantle beneath the Cathaysia Block to support the activity of mantle plumes related to the breakup of the supercontinents Nena/Columbia and Rodinia in Proterozoic time.

  8. Rock slope instabilities in Norway: First systematic hazard and risk classification of 22 unstable rock slopes

    NASA Astrophysics Data System (ADS)

    Böhme, Martina; Hermanns, Reginald L.; Oppikofer, Thierry; Penna, Ivanna

    2016-04-01

    Unstable rock slopes that can cause large failures of the rock-avalanche type have been mapped in Norway for almost two decades. Four sites have earlier been characterized as high-risk objects based on expertise of few researchers. This resulted in installing continuous monitoring systems and set-up of an early-warning system for those four sites. Other unstable rock slopes have not been ranked related to their hazard or risk. There are ca. 300 other sites known of which 70 sites were installed for periodic deformation measurements using multiple techniques (Global Navigation Satellite Systems, extensometers, measurement bolts, and others). In 2012 a systematic hazard and risk classification system for unstable rock slopes was established in Norway and the mapping approach adapted to that in 2013. Now, the first 22 sites were classified for hazard, consequences and risk using this classification system. The selection of the first group of sites to be classified was based on an assumed high hazard or risk and importance given to the sites by Norwegian media and the public. Nine of the classified 22 unstable rock slopes are large sites that deform inhomogeneously or are strongly broken up in individual blocks. This suggests that different failure scenarios are possible that need to be analyzed individually. A total of 35 failure scenarios for those nine unstable rock slopes were considered. The hazard analyses were based on 9 geological parameters defined in the classification system. The classification system will be presented based on the Gamanjunni unstable rock slope. This slope has a well developed back scarp that exposes 150 m preceding displacement. The lateral limits of the unstable slope are clearly visible in the morphology and InSAR displacement data. There have been no single structures observed that allow sliding kinematically. The lower extend of the displacing rock mass is clearly defined in InSAR data and by a zone of higher rock fall activity. Yearly

  9. Grinding into Soft, Powdery Rock

    NASA Technical Reports Server (NTRS)

    2004-01-01

    This hole in a rock dubbed 'Clovis' is the deepest hole drilled so far in any rock on Mars. NASA's Mars Exploration Rover Spirit captured this view with its microscopic imager on martian sol 217 (Aug. 12, 2004) after drilling 8.9 millimeters (0.35 inch) into the rock with its rock abrasion tool. The view is a mosaic of four frames taken by the microscopic imager. The hole is 4.5 centimeters (1.8 inches) in diameter. Clovis is key to a developing story about environmental change on Mars, not only because it is among the softest rocks encountered so far in Gusev Crater, but also because it contains mineral alterations that extend relatively deep beneath its surface. In fact, as evidenced by its fairly crumbly texture, it is possibly the most highly altered volcanic rock ever studied on Mars.

    Scientific analysis shows that the rock contains higher levels of the elements sulfur, chlorine, and bromine than are normally encountered in basaltic rocks, such as a rock dubbed 'Humphrey' that Spirit encountered two months after arriving on Mars. Humphrey showed elevated levels of sulfur, chlorine, and bromine only in the outermost 2 millimeters (less than 0.1 inch) of its surface. Clovis shows elevated levels of the same elements along with the associated softness of the rock within a borehole that is 4 times as deep. Scientists hope to compare Clovis to other, less-altered rocks in the vicinity to assess what sort of water-based processes altered the rock. Hypotheses include transport of sulfur, chlorine, and bromine in water vapor in volcanic gases; hydrothermal circulation (flow of volcanically heated water through rock); or saturation in a briny soup containing the same elements.

    In this image, very fine-grained material from the rock has clumped together by electrostatic attraction and fallen into the borehole. NASA/JPL/Cornell/USGS

  10. A Tale of Two Orogens: Comparing Crustal Processes in the Proterozoic Trans-Hudson and Grenville Orogens, Eastern Canada

    NASA Astrophysics Data System (ADS)

    Darbyshire, F. A.; Bastow, I. D.; Gilligan, A.; Petrescu, L.

    2016-12-01

    The Precambrian core of North America is an assemblage of Archean cratons and Proterozoic orogenic belts, preserving over 3 billion years of Earth history. Here we focus on two of the largest collisional orogens, using recent and ongoing seismological studies to probe their present-day structure and tectonic history. The 1.8 Ga collision between the Western Churchill and Superior cratons, along with microcontinental and island arc terranes, formed the Trans-Hudson Orogen (THO), a collisional belt similar in scale and shape to the present-day Himalaya-Karakoram-Tibet Orogen (HKTO). In the Mesoproterozoic, a series of collisions reworked the SE margin of the Superior craton and added new material over a period of several hundred Ma, culminating in the Grenvillian orogeny and the assembly of the supercontinent Rodinia. The Grenville Orogen is thought to have been a large, hot, long-lived plateau which subsequently underwent orogenic collapse. While similar in spatial scale, the Trans-Hudson and Grenville Orogens have significantly different tectonic histories, notably in terms of longevity and tectonic evolution. Comparison of these collisional belts with each other, and with the HTKO, provide valuable insights into plate-tectonic history. Recently a number of broadband seismograph installations have allowed a detailed study of present-day crustal structure beneath the THO and the Grenville. Receiver-function and surface wave studies provide information on crustal thickness variations, bulk crustal composition and crustal heterogeneity. The crust beneath the orogens is generally thicker, more mafic and more heterogeneous than that beneath neighbouring Archean and Phanerozoic domains, with significant along-strike variability and Moho complexity. We review and interpret the new crustal structure information in the context of the tectonic processes affecting the two contrasting orogens.

  11. Tracing Life in the Earliest Terrestrial Rock Record

    NASA Astrophysics Data System (ADS)

    Lepland, A.; van Zuilen, M.; Arrhenius, G.

    2001-12-01

    The principal method for studying the earliest traces of life in the metamorphosed, oldest (> 3.5 Ga) terrestrial rocks involves determination of isotopic composition of carbon, mainly prevailing as graphite. It is generally believed that this measure can distinguish biogenic graphite from abiogenic varieties. However, the interpretation of life from carbon isotope ratios has to be assessed within the context of specific geologic circumstances requiring (i) reliable protolith interpretation (ii) control of secondary, metasomatic processes, and (iii) understanding of different graphite producing mechanisms and related carbon isotopic systematics. We have carried out a systematic study of abundance, isotopic composition and petrographic associations of graphite in rocks from the ca. 3.8 Ga Isua Supracrustal Belt (ISB) in southern West Greenland. Our study indicates that most of the graphite in ISB occurs in carbonate-rich metasomatic rocks (metacarbonates) while sedimentary units, including banded iron formations (BIFs) and metacherts, have exceedingly low graphite concentrations. Regardless of isotopic composition of graphite in metacarbonate rocks, their secondary origin disqualifies them from providing evidence for traces of life stemming from 3.8 Ga. Recognition of the secondary origin of Isua metacarbonates thus calls for reevaluation of biologic interpretations by Schidlowski et al. (1979) and Mojzsis et al. (1996) that suggested the occurrence of 3.8 Ga biogenic graphite in these rocks. The origin of minute quantities of reduced carbon, released from sedimentary BIFs and metacherts at combustion steps > 700 C remains to be clarified. Its isotopic composition (d13C from -18 to -25%) may hint at a biogenic origin. However, such isotopically light carbon was also found in Proterozoic mafic dykes cross-cutting the metasedimentary units in the ISB. The occurrence of isotopically light, reduced carbon in biologically irrelevant dykes may indicate secondary graphite

  12. Lithospheric discontinuities beneath the U.S. Midcontinent - signatures of Proterozoic terrane accretion and failed rifting

    NASA Astrophysics Data System (ADS)

    Chen, Chen; Gilbert, Hersh; Fischer, Karen M.; Andronicos, Christopher L.; Pavlis, Gary L.; Hamburger, Michael W.; Marshak, Stephen; Larson, Timothy; Yang, Xiaotao

    2018-01-01

    region of mechanically and chemically rejuvenated mantle that was likely emplaced during late Precambrian/early Cambrian rifting. These observations suggest that the lithospheric structure beneath the Reelfoot Rift may be an example of a global phenomenon in which MLDs act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath.

  13. Aram Chaos Rocks

    NASA Technical Reports Server (NTRS)

    2005-01-01

    8 September 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows outcrops of light-toned, sedimentary rock among darker-toned mesas in Aram Chaos. Dark, windblown megaripples -- large ripples -- are also present at this location.

    Location near: 3.0oN, 21.6oW Image width: width: 3 km (1.9 mi) Illumination from: lower left Season: Northern Autumn

  14. Rover, Airbags, & Surrounding Rocks

    1997-07-05

    This image of the Martian surface was taken by the Imager for Mars Pathfinder (IMP) before sunset on July 4, 1997 (Sol 1), the spacecraft's first day on Mars. The airbags have been partially retracted, and portions the petal holding the undeployed rover Sojourner can be seen at lower left. The rock in the center of the image may be a future target for chemical analysis. The soil in the foreground has been disturbed by the movement of the airbags as they retracted. http://photojournal.jpl.nasa.gov/catalog/PIA00619

  15. Exercise Desert Rock, Staff Memorandums. Army, Camp Desert Rock, Nevada.

    DTIC Science & Technology

    1957-01-01

    I AD-AGAG 257 EXERCISE DESERT ROCK LAS VEGAS NV F/6 IS/ 3 EXERCISE DESERT ROCK, STAFF MEMORANDUMS. ARMY. CAMP DESERT ROCK-ETClUlCASIFE mm95i mm... Exercise Safety Progra - . 1. PUrose: To establish ane’ffective safety progr.Rm toreduce, and keep to a minimum, accident,1 manpower and monetary losses. at...agencies will be- followed. Supervispry personnel will: become familiar with those that Pre applicable to thei£r... operations. The Exercise Safety

  16. High-K calc-alkaline magmatism at the Archaean-Proterozoic boundary: implications for mantle metasomatism and continental crust petrogenesis. Example of the Bulai pluton (Central Limpopo Belt, South Africa)

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Martin, Hervé; Doucelance, Régis; Moyen, Jean-François; Paquette, Jean-Louis

    2010-05-01

    arc). Geochemical modeling indicates that the mafic facies of the Bulai pluton can be achieved by a two-stage process: 1) a liquid produced by melting of subducted terrigenous sediment is entirely consumed by metasomatic reactions with mantle peridotites, producing a metasomatic amphibole- and phlogopite-bearing assemblage ; 2) Low-degree melting of this metasomatized mantle gives rise to magmas with the same trace element signature than the Bulai mafics. Such a petrogenetic model is consistent with previous geochemical and experimental study about sanukitoids, which concluded that these magmas result of interactions between slab melts and the overlying mantle wedge. Subsequently, at crustal levels, the Bulai monzodiorite not only underwent significant fractional crystallization, but also induce melting of the host rocks, and mixed with the resulting felsic crustal magmas. Such that it can be concluded that the whole Bulai suite can be derived through AFC processes. Our geochemical study failed in demonstrating any significant role played by melting of subducted metabasalts, which contrasts to Archaean times, and would point to lower thermal regimes as the wet solidus of metasediments is ~ 50 to 100°C lower than that of metabasalts at slab pressures. In addition the fact that the whole felsic melts were consumed by reaction with peridotite is indicative of lower degrees of melting of the slab compared with Archaean processes. Both conclusions imply that the geodynamic changes that took place at the Archaean-Proterozoic transition and witnessed by sanukitoid-related rocks are the result of progressive and global cooling of Earth.

  17. Facies and age of the Oso Ridge Member (new), Abo Formation, Zuni Mountains, New Mexico

    Armstrong, A.K.; Stamm, R.G.; Kottlowski, F.E.; Mamet, B.L.; Dutro, J.T.; Weary, D.J.

    1994-01-01

    The Oso Ridge Member (new), at the base of the Abo Formation, nonconformably overlies Proterozoic rocks. The member consists of some 9m of conglomerate and arkose composed principally of fragments of the underlying Proterozoic metamorphic rocks; thin, fossiliferous limestone lenses are interbedded with the arkose. Biota from the lenses include a phylloid alga, foraminifers, conodonts, brachiopods, and molluscs. The age of the Oso Ridge Member is Virgilian Late Pennsylvanian) to Wolfcampian (Early Permian). -from Authors

  18. Apennine Front revisited - Diversity of Apollo 15 highland rock types

    NASA Technical Reports Server (NTRS)

    Lindstrom, Marilyn M.; Marvin, Ursula B.; Vetter, Scott K.; Shervais, John W.

    1988-01-01

    The Apollo 15 landing site is geologically the most complex of